Brown Bear, Brown Bear, What Do You See? and other similar Bill Martin books.
My son is just about 5 months old and will totally tune into on these books (has since about 3.5 months). I think because the images are big, simple and colourful (Eric Carle is the illlustrator) and the prose is repetitive.
I'm a teacher (and was a teacher-librarian) and found that most children love these books up to age 6 or 7.