For author Joan Ohanneson, the library was her monastery these past 10 years as she tried to make sense of a 12th-century whirlwind named Hildegard of Bingen. Hildegard was a cloistered nun whose spirit soared to the galaxies. How do you explain it? She was a visionary and mystic, a prophet, composer, herbalist and healer. Living on a pristine, green mountaintop in Germany 900 years ago, she envisioned looming ecological disaster. At a time when women were confined to life's periphery, she counseled kings, brashly challenged the church's patriarchal hierarchy, predicted the deaths of corrupt bishops, and established a Christian vocabulary for speaking about the feminine side of God.

"She's too much," says Ohanneson, shaking her head, amused and befuddled at the thought of Hildegard's voluminous musical compositions, works on holistic medicine, theological tomes and rhapsodic visions, all these texts "so dense, you feel like you're just walking through honey, trying to get a handle on her ideas. You get spiritual indigestion, working with Hildegard."

And now there is "Scarlet Music: Hildegard of Bingen, A Novel" (Crossroad, $14.95), Ohanneson's meticulously researched, fictionalized life of the medieval abbess, whose 81-year saga, full of sadness and ecstasy, achievement and contradiction, sneaked up on Ohanneson and absorbed her attention for a decade.

For years now, the Catholic-raised Ohanneson, a longtime resident of San Jose's Willow Glen, has talked about and written about her "lover's quarrel" with Catholicism over its "exclusion of women from full participation in the Church." In Hildegard, she stumbled upon a woman who both embraced and transcended doctrine, who spoke of the primacy of the heart at a time when scholasticism - representing the primacy of the intellect - was establishing itself in Europe. Today, Ohanneson is convinced that organized religions are failing, that unbridled intellect is again suspect, and that Hildegard's heart-felt, faith-filled messages speak boldly in a new era of spiritual excitement.

Hildegard "has come back," Ohanneson says, to warn against ecological destruction ("the fruit shriveling on the tree with poison sweetness," Hildegard wrote, "the air choking the people with filth") and to preach her message about God's feminine aspect. At a time when the cult of the Virgin was rising all around her, Hildegard tapped into the older, feminine personification of the divine, known as wisdom or Sophia, who pervades the Hebrew Scriptures.

"Hildegard has returned," Ohanneson says, "to resurrect ' Lady Wisdom.' To give us back ourselves. To give men the part of themselves they are sometimes loathe to own. To give women that spiritual self-respect without which you can't breathe."

She has also come back, in Ohanneson's estimation, to call attention to another ongoing problem of the church. The author mentions last week's $120 million civil court judgment in Dallas against the Roman Catholic diocese and a priest accused of sexually abusing boys. In that judgment, she hears an "echo" of Hildegard who "publicly called priests of her time 'fornicating adulterers who ravish the church with greed.' Unbelievable."

But Hildegard wasn't a one-dimensional truth teller, and "Scarlet Music" - the title derives from Hildegard's description of the blood of Christ - tells her whole, complicated story. Yes, she described precious human life as a "feather on the breath of God," and each of her songs, to use Ohanneson's words, is a veritable "leap to heaven."

But as nest year's 900th anniversary of her birth approaches, as Hildegard CDs sell in the tens of thousands, attempts to market her as a medieval rock star or forerunner of the New Age ring falsely beside the reality of her life.

Hildegard was a noblewoman who healed paupers, it is written, but she was also a defender of the faith who publicly excoriated heretics. Accused of megalomania for manipulating her nuns and other fellow clergy, she didn't mince words, and once called Frederick Barbarossa, Emperor of the Holy Roman Empire and her former advocate, a "madman." She was filled with a fire reminiscent of the Hebrew Prophets, warning corrupt clergy about their inevitable deaths: "Your days are numbered!"

"I said to someone, ' Would you like to have her for dinner?' " Ohanneson says with a laugh. "She said, ' I'm scared to death of her.' "

Hildegard taught that "surprise" is another name for "revelation," and Ohanneson's collision with Hildegard was both surprise and revelation. Sitting in her living room, talking for hours about the woman who became her obsession, she still seems surprised by the twists and turns of Hildegard's life.

Ohanneson, 66, married 42 years and mother of four daughters, has long been something of a radical Catholic. At the Jesuit School of Theology in Berkeley in the early '70s, she taught a course on women in the church and society. She wrote two books: "And They Felt No Shame: Christians Reclaim Their Sexuality" and "Woman: Survivor in the Church."

"The spiritual wellspring for everything I do comes from my tradition," she says. But she knew next to nothing about Hildegard, who had been promoted unsuccessfully for canonization by several popes, then disappeared into medieval martyrologies until her discovery by modern academics.

Then in the late 1980s, Ohanneson was suddenly and irrationally seized, she says, with the idea of writing about Hildegard's life. She started hearing whisperings, in conversations and writings, about Hildegard, and told her husband, Greg, that they needed to visit Hildegard's original Benedictine monastery at Disibodenberg, Germany, near the town of Bingen: "I said, 'I want to walk where she walked and I want to see the sunsets that she saw,' and he said, 'Gimme a break. Let's go to Florence.' "

They went to Germany, where Ohanneson found out that a former colleague was producing the first German television documentary on the life of Hildegard. Soon Ohanneson was walking on the Disibodenberg mountaintop, standing in tears beside the tiny excavated plot that had been Hildegard's room. At Eibingen, nearby on the Rhine, she visited the still-flourishing monastery that Hildegard founded in her latter years and requested a meeting with one of the world's foremost Hildegard scholars. Ohanneson expected the brush-off from Sister Adelgundis - "a brisk, efficient little bit of a thing in a habit." Instead the scholar encouraged her fledgling project and Ohanneson was increasingly confused: "You're catapulted into something and you think, 'What am I doing here?' "

All of these events were "like a silk thread pulling me along," Ohanneson says, and the outlines of Hildegard's life became clearer.

Hildegard, born in 1098, was the tenth child of noble parents. Tithed to the church, as was the custom of the day, she was placed in the care of Lady Jutta of Sponheim, the anchoress of Disibodenberg monastery, where they lived an almost solitary life in one room with one window overlooking the church.

Young Hildegard was given a rudimentary education by the anchoress. Clearly the girl was different: From the age of 3, she later wrote, she had "seen a Light so intense that my soul trembles." But her visions were her secret: "She 'stuffed them,' as we say today," Ohanneson says. The result was that Hildegard, isolated and lonely, was tortured by self-doubt and debilitated by depression and illness. This began to change when, at age 42, she had an awakening. As Hildegard described it, she was instructed by Lady Wisdom - her lifelong "guide through the labyrinths," Ohanneson says - to record her visions. And thenceforth, for decades, she dictated her thoughts and premonitions about the Holy Spirit, Lady Wisdom, and the "radiance of creation."

"I crawled on my hands and knees for 10 years, just trying to make sense of the material...the enormity of her accomplishments," Ohanneson says. "I had to essentially take on the 12th century. The socio-political - all those popes and anti-popes."

Ohanneson lived in libraries, struggled through translations, consulted with experts on Hildegard and the Middle Ages. Did 12th century people eat with forks? Did they use salt? Did the women curtsy?

"I wanted to be authentic in terms of medieval history," Ohanneson explains. "And I wanted it to be rich and full, a real tapestry of the times, against which I could place Hildegard and see her struggles, and what the gender roles were, and the classist dimensions. I had to learn the Rule of St. Benedict and the structure and vocabulary of the Divine Office. I spent months just on Barbarossa."

"It was a great ride, but the main engine was this passion I had for this woman. And I was by turn angered and elated and skeptical and humbled by her."

Hildegard had become Ohanneson's ticket to seeing afresh the possibilities that life offers. How could one person, largely uneducated, abandoned by her parents, tormented by doubts and sickness, accomplish so much? After chasing Hildegard of Bingen for a decade, Ohanneson finds that she is still "racing ahead of us."

"You know, Tennessee Williams has this great quote: 'To create is sacred, because it's all we know of God.' There's something here that's bigger than I am," Ohanneson says.

"She sits at my breakfast table every morning - Hildegard sits there. She's with me all of the time. I ask her to take care of people I love and for whom I promised to pray. I ask her for wisdom and light...because her whole life was a triumph over self-doubt. No matter how many accomplishments she had, no matter how many barriers she broke, every time she took another risk, every time she struggled, she had to climb the mountain of self-doubt again. And who of us hasn't wrestled with that demon?"