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.
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.
Justin Colussy-Estes / Guys Lit Wire