As a friend to the fertile, I've been to a bunch of baby showers over the past few years, and it seems to be a hot trend among the expecting to ask that shower attendees bring one of their favorite children's books, sometimes in lieu of a registry (at least among those who can afford all the baby stuff themselves and are just having the shower as an excuse to see their friends and eat cake before they enter baby jail for the next few years). It's a sweet idea, especially because most of us have a unique, powerful nostalgia for the books we loved when we were small, but we also don't usually have the most thorough memories from that age, so we only remember the classics, and the mom-to-be ends up with five copies of In The Night Kitchen. Without getting into specific titles (since your friend is probably not interested in children's books beloved by a random person on the internet), here are some guidelines for picking books that are meaningful, but won't end up plentiful.
Get Regional: Since most of my friends from New York left the city to spawn, I look for NYC-based picture books, like the classic This Is New York, by M. Sasek, from 1960 (reprinted in 2003). Sasek did books for other major cities, too, and his illustrations are stunning, colorful, and delightfully retro. There are also other random books out there that skew even younger and use the numbering and lettering of the MTA to teach the ABCS/123s.
Specific Illustrators: Everybody knows about Eric Carle's The Very Hungry Caterpillar, or at least enough people do to risk your gift turning into the back-up copy, or a coaster. Luckily, Carle has done loads of other beautiful books on other creatures—from sea horses to sloths—that are just as adorable and fun to look at. Similarly, most adults are familiar with Maira Kalman's illustrations for The New York Times or her TED talk, but she's a brilliant author and illustrator of children's books as well. While most are out of print, they're found easily enough on Amazon. Her series on Max the dog poet (and his love, a Dalmatian named Crepe Suzette!) is exquisite.
Skew Older: While a baby shower is technically a celebration for a baby, it's never a good thing when all the mom-to-be gets are clothes, books and accessories that cater to a 0-6 month old. Babies need new crap all the time as they zip through the stages of development, so, as you would hopefully get onesies that are size 12-18 months, you can also get books that are better for a kid at 5 or 6. After all, if there's a book you remember well that meant a lot to you, even if it's not a picture book, it's better to choose that than something pretty you have less of a connection to.
If you must get Maurice Sendak, get the Nutshell Library four pack with Alligators All Around, Chicken Soup with Rice, One Was Johnny, and Pierre.
They're not big picture books, but superior bedtime reading, and enough with the Wild Things already.