There are so many wonderful books for preschool aged children! If your child attends preschool or childcare, try asking his or her teachers to recommend favorites, and also check in with the children's librarian at your public library.  There are many lists (see link below) of recommended books, but here are a few of my favorite books and authors.   I've taught preschool through 1st graders for the last 18 years, these have all been winners:

Brown Bear Brown Bear, Bill Martin Jr.
Time to Sleep, Denise Fleming,
Saturday Night at the Dinosaur Stomp, Carol Diggory Shields
To Market to Market, Anne Miranda
Who is the Beast?, Keith Baker
Puff the Magic Dragon, Peter Yarrow
The Dragons are Singing Tonight, Jack Prelutsky
The Magic Hat, Mem Fox
"Slowly, Slowly, Slowly," said the Sloth, Eric Carle
No Matter What, Debi Gliori
Fish Eyes, Lois Ehlert
Baby Food, Saxton Freymann
Sharks, Gail Gibbons