I recommend all the books by Claude Piron. I haven't read them all yet, but Claude Piron had a style very simple and rich ! He is really the author who gave me a crush on esperanto ! He was able to use all its particularities and make it so easy and subtile in the same time ! It's so delicious ! I suggest : « Lasu min paroli plu », which is written for beginners with a slow progression. Or « Dankon amiko ». Also « Gerda malaperis » is very known but it's not my favourite !! You can find many of them in PDF there : http://claudepiron.free.fr/livres.htm

About Esperanto itself, every esperantist should read « la bona lingvo ». But it's not for beginners.