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.
Esperanto (Bookshelf) — another list of Esperanto books on PG.
Project Gutenberg E-texts in Esperanto — another short list of PG books.
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! Dictionary — with multiple languages.
Being Colloquial in Esperanto: A Reference Guide, David K. Jordan — Online version of Jordan's excellent book.
Tekstaro de Esperanto — A searchable text corpus of the Esperanto language; a large collection of very diverse texts for linguistic research on Esperanto. Words and phrases may be entered to show usage in the texts.
Plena Manlibro de Esperanta Gramatiko — Complete Manual of Esperanto Grammar. Esperanto grammar in an easy-to-learn format.
Dr. Esperanto's International Language (La Unua Libro, 1887) — Translated by R. H. Geoghegan [HTML transcript]
An Attempt towards an International Language (La Unua Libro) — Translated by Henry Phillips Jr. [transcript]
Aldono al la Dua Libro de l' Lingvo Internacia (1889) [HTML transcript]
Fundamenta Krestomatio de la Lingvo Esperanto (1903) [transcript]
Fundamento de Esperanto (1905) [formatted HTML version]
Gramatiko (English section)
Ekcercaro (orig. 1894)
Universala Vortaro (orig. 1893)
Deklaracio pri la Esenco de la Esperantismo (Declaration of Boulogne, 1905)
Pri Reformoj en Esperanto (1907)
Proverbaro Esperanta (1910) [transcript]
Kongresaj Paroladoj (Paroladoj de Zamenhof, 1905–1913) [transcript]
Lingvaj Respondoj (1925) [partial transcript]
Poemoj (Poems, various dates) [transcript]
La Batalo de l' Vivo (The Battle of Life: A Love Story by Charles Dickens, tr. 1891) [HTML transcript]
Hamleto, Reĝido de Danujo (Hamlet, Prince of Denmark by W. Shakespeare, tr. 1894) [HTML transcript]
La Revizoro (The Government Inspector by Nikolai Gogol, tr. 1907) [transcript]
Georgo Dandin (George Dandin by Molière, tr. 1908)
Ifigenio en Taŭrido (Iphigenia in Tauris by Goethe, tr. 1908) [HTML transcript]
La Rabistoj (The Robbers, Friedrich Schiller, tr. 1908)
Marta (by Eliza Orzeszkowa, tr. 1910) [HTML transcript]
Fabeloj [Libro 1] [Libro 2] [Libro 3] [Libro 4] (Fables by Hans Christian Andersen, tr. 1917) [transcript]
La Malnova Testamento (The Old Testatment of the Bible, 1925) [HTML transcript]
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 a million Esperanto students on Duolingo.