My perfect book exists on many levels. It's rich and steeped in insights, but the language is facile without losing lyricism. It's about someone who isn't perfect when the book begins, but who is broken in interesting ways, and who isn't perfect when the book ends either, but is even more interesting and maybe a little wiser. It has adventure and derring-do and characters I believe in, and a little mystery, and no stupid romances for their own sake, and it probably has a death-or-glory stand. It's honest about the ugly bits, because that makes the lovely bits stronger. It's salty and funny and sweet and it's true, and it has me crying on one page and laughing on the next.
And it probably has a harpy in it. Or, failing that, a dragon.
I've found a lot of books that are close to my perfect book. So far, The Last Unicorn is the nearest thing I've found.