I was going to suggest Swallows and Amazons by Arthur Ransome as one of my favourite (series of) books, but I see jenn_calaelen beat me to it. Hence I will go with another rather obvious book and say To Kill a Mockingbird by Harper Lee. It really is a wonderful, and positive book, showing that humans can and will rise to better themselves if we believe in them and give them a chance. When my friend K mentioned that he hadn't knowingly read many books by female authors this was my immediate recommendation. I then bought it for him for his birthday, just to make sure he got the hint! Happily he does have good taste in books and enjoyed it very much.

Edited to fix html-coding error.