Above was actually his 5th birthday, silly granny!
Doctor Suess books are great, as are the Oxford Reading Tree non-fiction series 'Floppy's Phonics'. Moonlight Publishing's series 'My First Discoveries' are a beautiful non-fiction series in a good sized format for small hands. They make clever use of overlays and stand up well to handling, some we've had for 10 years. The Weather, The Body, Trains are favourites.
For bedtime reading there's Shirley Hughes' Alfie books, Mick inkpen's Kipper, several Allan Ahlberg favourites and the personalised 'The Little Boy Who Lost his Name'.
My GDs have very much enjoyed Jacqueline Wilson's books, they accept that not all families are snugly nuclear and conventional.
With such a huge amount of books available its sometimes difficult to keep up. I like my GC to have the original AA Milne Winnie the Pooh books and his poetry books, Alice with illustrations and an illustrated A Christmas Carol together with children's books with the stories of Shakespeare's plays, Classical Myths, Fairy Stories and Folk Stories. Just be careful to get a good version of the last four, they vary so much.