World of Books have a great selection of second hand books for children. I have bought heaps for my grand daughter although she is only 10 months! The Works also have great selection of books usually 10 for £10. Lots of books on the matket about starting school which can help allay any worries wee ones have. As former primary teacher - nursery and infants- I would steer clear of the phonics, maths, handwriting, etc books aimed at 4-5 year old. There is plenty to talk about in ordinary story books to cover these early skills. Far more important he can dress and undress for PE, use cutlery, go to the lavatory independently, have good manners, listen to and follow instructions, etc. My boys were lucky as where we live in Scotland the earliest children can start P1 is 4 yrs 6 months and this year all children can have an additional year in nursery and start in P1 Aug 2024 (previously it was only those who were 5 in the Jan or Feb who automatically had this option).