I always liked the hardback editions of children’s books in UK libraries in the 70s and 80s , and have sought them out secondhand since. Favourites were the US Danny Dunn science story series - almost all of which were amazingly available in the UK, Dr Who hardback editions of the famous Target novelisations - pre-vcr the only way to access old stories (even in the Uk these were very rarely repeated). Hugh Walters 1960s/1970s sf books (passage to Pluto, journey to Jupiter, expedition Venus etc.)
Also the Agaton Sax comedy detective stories, and the largely unknown Uncle the elephant books by JP Martin.