Well, it's not a novel and maybe this isn't what you're looking for but: When I was in grade school I bought a little book called Sam the Cat Detective at a book fair. It's classic noir private eye style, except the detective is (obviously) a cat. It's really cute and surprisingly well written and I kept that copy all my adult life, revisiting it now and then.

After my house burned down last April I was overwhelmed with the idea of restarting my library, since I re-read my favorites fairly often. But I didn't know what to purchase first. Eventually I threw away the idea of getting any 'grown-up' books (I'm 40) right away, looked for Sam on thriftbooks and discovered to my delight that there were two sequels! So now I have all three.