I couldn't count the number of times I've told someone, "reading is the best thing you could do with your child." On the technical side, it fosters creativity, develops imagination, increases vocabulary, and when done properly, makes it so you never have to force a child to sit down to teach them something, because they've already learned it in a book and the subsequent interactions that stem from co-reading. I've been asked, and had to ponder for myself: "Which book?" Sure, there are the classics, from Chicka-Chicka boom-boom (and ABC) to The Very Hungry Caterpillar. However, with some exceptions, anything you read to/with a child will be beneficial on most levels, and deleterious on very, very few. I tend to shy away from Disney books (or any other books with corporate agendas) and books that claim to help you teach your child (most of that happens naturally), but most books are fair game.
"Age-appropriate" is a term that gets used quite liberally, and for good reason. Focusing on materials (or clothes or words) that are age-appropriate ensures that we maximize a child's development through a certain activity, but no one in the world is surrounded with only "age-appropriate" things, and society still operates just fine (not really, but that's not an issue to be solved on a blog).