That being said, poetry in the form of nursery rhymes has always been popular.  Reading or reciting nursery rhymes to your baby is a wonderful way to expose them to language.  My favorite book of nursery rhymes would have to be by Mary Engelbreit.  Many nursery rhymes are actually awful to read.  They are not cheerful and light but dark and almost evil.  Mary Engelbreit has selected the best nursery rhymes and her illustrations are beautiful and appealing to young children.  If you choose to buy just one book of nursery rhymes, I would strongly recommend Mary Engelbreit's Mother Goose.

Another really great book of poetry to read a loud to your little ones is The Llama Who Had No Pajama.  This is a collection of poems by Mary Ann Hoberman.  She is a wonderful children's author and her poems are just delightful.  This is another book worth owning.  You can read a few short poems to your baby each day.  Children are never too young to be read to and reading poetry to your baby is a wonderful way to develop their language abilities and their vocabulary.

I have three other poetry books that I really like.  My children have gotten a lot of use out of these next three books.  I have them memorize poems of their choice so they can recite them from memory.  If you have to choose only one book of poetry, I would recommend Favorite Poems Old and New.  It is very thick and includes the best poems by the best poets.  It will give you plenty to read. 

If you want a small book that is easy to handle but has a collection of wonderful poems you may want to check out A Treasury of Best Loved Children's Poems or The Random House Book of Poetry For Children.