Most of the time, I think, we find books by recommendation. But there's nothing better than finding that book that grabs us with its great cover, and the pages inside live up to the image's promise.
Like the cover to The Mysterious Benedict Society, by Trenton Lee Stewart. That cover by Carson Ellis has just the right amount of intricacy and wackiness to suggest just what kind of a fun, mysterious, and delightful book this is--offbeat, very smart, and full of fun.
Carson Ellis also does the interior art (chapter headings, etc.) which amps up the interplay between good book and good complementary art. I can think of two books that I discovered and loved over the years that might not have stuck out in my mind if it weren't for the spot illustrations inside.
Sharon Creech's The Wanderer is a book I go back to again and again. I love the interplay between two narrators, and the constant questions the book makes you ask: which narrator can I trust and what is the truth? But what blew me away was David Diaz's amazing spot illustrations. Simple black shapes that define objects, but in a way that evokes those same questions: what is this thing I'm looking at? Can I trust what I see? Truly great stuff.
Similarly, The House with a Clock in its Walls by John Bellairs was very fun when I discovered it as a pre-teen--full of creepy mystery. Later I re-read it and discovered that the book's hero, Lewis Barnavelt, is something of a whiner. But the world Bellairs created, with hints of magic and horror and tragedy, is worth going back to. And the book transcends the drawbacks with the fantastic illustrations by Edward Gorey, whose name is almost synonymous with all that and more: the mere presence of a Gorey image in a book sets a tone like a soundtrack. You immediately get a sense that the book is going to be humorous, dark, maybe grotesque and possibly filled with terrible people.
Of course, sometimes this match of artist and writer can backfire. For instance, the awesome cover illustration Jordan Crane did for Matt Haig's The Dead Father's Club had me begging to borrow a friend of mine's copy. Unfortunately, she had the paperback edition, which has a different cover. No insult to the artist, but that's now further down in my pile of books to read...