Esperantaj Libroj — Books in PDF format. e-espero.info
La elektronikaj libroj de FEL — ebooks in ePub, Mobipocket, and PDF. Flandra Esperanto-Ligo (FEL).
Esperanto books — various formats. Project Gutenberg.
Bitoteko — Books in PDF format. Hispana Esperanto-Federacio.
A First Reader: Intended for those who are beginning the study of Esperanto by E. A. Lawrence.
Plena Ilustrita Vortaro de Esperanto — The exhaustive dictionary of Esperanto.
Reta Vortaro — usage and translation dictionary.
lernu! — lernu! is a free website for getting to know and learn the international language, Esperanto. It's also a sort of experiment in learning languages online, without a teacher, and without classes. There are nearly a quarter million users.
Esperanto on duoLingo — A fun and easy way to learn languages. There are currently over half a million Esperanto students on duoLingo.