This is not a book to read when you want to read a book like another kind of book. It should be shelved under "literature and fiction" because the people who go to the science fiction and fantasy section are not looking for a book like this, though some of them will be very happy when they find it. Douglas Lain is a great writer, but when I try to figure out how to recommend this book, I'm at a loss, because Billy Moon's characteristics are strengths or weaknesses depending on what you're looking for. I searched for other reviews to see if I could simply recommend one, and I came away thinking they're all fair, the positive ones and the negatives ones. Trying simply to be as clear as possible, I'll say that for me, the book is fascinating without being compelling, so I read it in bits over several weeks. Whether I would've found it compelling if I'd taken the time to read it quickly I don't know, but it isn't the sort of book that makes me rearrange my schedule or stay up all night. It's a book about love and honor and responsibility and reality and family and revolution that looks at those things quietly and quirkily. Lain reminds me a little of Chesterton and Vonnegut in ways I can't explain. Here's a review that may be as good as any if you want to know a little more: Douglas Lain's 'Billy Moon' | Portland Monthly.
ETA: For a couple more takes by readers with different tastes:
Book Review: ‘Billy Moon’ by Douglas Lain | Blogcritics
Book Review: Billy Moon by Douglas Lain | Marxist-Humanist Initiative
ETA 2: Library Review gave it a lovely writeup and made it their Debut of the Month: Science Fiction & Fantasy Reviews | August 2013.