Midway through RIDING THE BUS WITH MY SISTER, Rachel Simon's engrossing, nontraditional memoir, Simon makes a startling observation: Almost all of the characters with mental retardation depicted in books come from fiction. FLOWERS FOR ALGERNON, OF MICE AND MEN, THE SOUND AND THE FURY --- she ticks them off one by one but can't remember a work of significance, or even one of dubious merit, that is nonfiction. Perhaps Simon has written the first book in this cannon.

RIDING THE BUS WITH MY SISTER has the insight of a literary work, the angst of a memoir, the heroes of a novel, and the page-turning sensibilities of a thriller. There is little suspense or drama, at least of the broader scale type. But for those who want to read about true life, it doesn't get any more real than dealing with a disabled member of the family --- Rachel's younger sister Beth, to be specific. She's a stubborn, independent, mildly retarded woman of 38 when Rachel agrees to take time out of her busy and mostly empty life to ride the buses with her sister. Beth, who lives alone in an assisted living facility, rides the buses in her mid-sized Pennsylvania city from dawn to dusk everyday except Sunday (no runs). She loves bright colors, brighter clothing, Top Forty radio and her boyfriend, Jesse. Beth hates mean people, vegetables, bigots and dentists. For sure, she is not your typical protagonist and neither, thank goodness, is Simon your typical narrator.

She admits to many faults, to being too wrapped up in her work to spend time with people, much less her troubled and troubling sister. Rachel undertakes the year of bus riding, scrunched between daily commitments to writing, teaching and planning events at a bookstore, in order to grow closer to Beth but also to assuage her guilt at being a bad sister. She remembers a time when her father, desperate after months of a then-mid 20s Beth acting out in ways he could not control, asked her to take temporary custody of her younger sister one weekend a month. Rachel refused, as did siblings Laura and Max. Though Rachel and Beth played together when they were young, their conversations have become forced and uncomfortable --- until Rachel agrees to ride the buses.

There she finds that Beth has fashioned a community for herself. Not every driver likes her. Some won't even tolerate her. Unsympathetic passengers sometimes scream at Beth, an unstoppable chatterbox. Beth feels a sense of purpose and entitlement on these buses that both amazes and frustrates her sister.

Through their 12 months, Simon realizes, startlingly, that she does not know the actual definition of "mentally retarded." Her quest to learn more about her sister's disability is one of the most compelling parts of the book. Simon doesn't crusade or sugarcoat. She presents differing yet intriguing views on how those with mental retardation should be treated, eventually settling on self-determination as the imperfect right choice. No matter what, Simon never claims more knowledge than she has. We are learning right along with her. As the lacking literature on mentally disabled people shows, this is a good thing because we are mostly an uninformed audience.

The only real criticism of RIDING THE BUS WITH MY SISTER may be the way Simon raises the bus drivers to pedestals, in the way that Beth herself often does. They are portrayed as wise wageworkers, dispensing penny truths that lessen white-collar guilt during their many cameos. Simon seems to acknowledge that she is at risk for demigoding these regular humans when she revisits them later in the book, explaining some flaws that may have been overlooked upon first acquaintance. Of course, that's hard to fault when some of these too-good-to-be-true drivers seem exactly that, such as Jacob, the driver who tries to teach Beth the philosophy of "do unto others" and brings the sisters to the beach on his day off.

More than anything, though, it is the italicized pieces at the end of each section that bring the book to life. These are Simon's accounts of growing up with her sister, and they combine for a fascinating look at what it's like growing up in a family living with disability. No family can claim it is without distractions or the tiniest bit of dysfunction; Simon's clan has plenty of both. And yet in the end, there is a satisfactory, realistic ending. Beth will continue to ride the buses and Rachel will continue to wonder if she is doing right by her sister. But these days, at least the two understand each other slightly better.