I spy kids books (I think they call them Scholastic Readers). When they're small, you can put them in your lap and they can enjoy the books for colors and objects and patterns. As they get older, they'll help look for things. And when older still, they can learn to read. They're great for a wide variety of ages.

My daughter's favorites are the Maisie books. I really enjoy reading the Francis books, Harry the Dirty Dog, and anything by Steven Kellogg (except the Pinkerton book where the burglar holds a gun to the head of Pinkerton's owner).

In addition to board books, I found some longer books that have rhyming text to be a godsend for my infant daughter. Something about the rhythm of the way you would naturally read the words put her right to sleep.

Aside from the old standbys (Goodnight Moon, Brown Bear, etc.), some favorites in my house have been:

Tumble Bumble
Bear Snores On (or really any of the Bear books)
The How Does a Dinosaur... series
Curious George (recognizing that a lot of the concepts in this book are pretty out of date/bordering on offensive).
The Peter Rabbit books--my kid loves Two Bad Mice and Jemima Puddleduck; go figure.
That's Not My... series.

My most favorite book to read to my children is Iggy Peck Architect. It has a good story for small children (great rhyming pattern) and interesting for older children as well. Highly recommend!

OP here. Thanks for all the suggestions. It's fun to be reminded of some of the books I liked as a kid (LOVED The Monster at the End of This Book!).

To answer some of your suggestions, we are bringing ONLY books. It's a small group of friends, and it has become our tradition to do this when any of us has a baby.

I think this is one of the sweetest books out there and perfect for a shower gift:

http://www.amazon.com/Mother-Mine-Marion-Dane-Bauer/dp/0689822677

Also, my daughter who is now a toddler seems to love the animal pictures.

OT - I got an invitation that phrased the "bring a book thing" to imply that you're bringing a book in addition to another gift. I'm not going to quote it because they could read this.
Really? Is that a thing?

Here's a couple that are a little off the beaten path, and loved by my daughter and other kids:
"Is Your Mama a Llama?"
"The Monster at the End of This Book" (a sesame street classic)
"I Love You, Goodnight"
(and "Guess How Much I Love You")

Little Golden books. You can get them for about $4 a piece, they are classics and terrific books, and 5 of them would make a great set.

Another idea would be something like this: http://www.amazon.com/Very-First-Library-Eric-Carle/dp/0399246665/ref=sr_1_7?s=books&ie=UTF8&qid=1333384992&sr=1-7

Hug - Jez Alborough
Goodnight Gorilla - Peggy Rathmann

These were two of our faves!

I threw a bring-a-book shower earlier this year and it was fun to see what everyone brought. If you are bringing a registry gift in addition, just bring one book. If this is your gift, you can be creative.

If you plan to buy more common authors -- Carle, Boynton and so on, make sure you give a gift receipt so the new parents don't have 5 copies of Goodnight Moon.
Do you have any favorites from your childhood? Kind of like buying toddler size instead of newborn clothes as a gift, maybe books geared towards a preschooler? Think Dr. Seuss or series' like Madeleine or Olivia.

Some of my favorite books for my baby and young toddler weren't as common. I Love You, Little One. On the Night You Were Born. If the parents are at all religious, God Gave Us You is also beautiful. Now that my daughter is 2 she loves things that are focused on mommies and daddies loving their small children. She asks to read and re-read these.

Google the list "100 Books to Read Before Kindergarten" and pull a few ideas from that.

I like the Shel Silverstein, "Where the Sidewalk Ends" sometimes you can find it with a CD of the author reading some of the poems. It is nice because it will be great when the child is a little older, babies don't really "need" that many books.

Get some Sandra Boynton books. You can usually find them in sets. Or Dr. Seuss.

When my DD was little she also loved the kid's books that had shapes, colors, and numbers. You can get those kind of books at the dollar store.

Either way, keep the receipts because I will bet that there will be duplicates.

I have two book-themed baby showers to attend this week and am looking for some ideas. I've been to these things before and have been able to put together a nice collection, but I am on a major budget. I can't really spend more than $20 per set of books. Any recommendations?