📖 Textbooks

Beginner

Complete Esperanto is one of the most recommended textbooks for beginners.

Wikibooks Esperanto Textbook is a free textbook, written collaboratively.

Intermediary

Being Coloquial in Esperanto is aimed at English speakers who have completed an Esperanto introductory course.

Grammars

The PMEG (Plena Manlibro de Esperanta Gramatiko or Complete Manual of Esperanto Grammar) is considered by many as the main reference work of Esperanto grammar. It is written as a practical guide and avoids jargon as much as possible.

Lernu has an English version of the same book. This version uses more traditional grammar termilogy.

Wikibooks has Esperanto: A Complete and Comprehensive Grammar.

Also, Wikipedia has a very comprehensive article on Esperanto grammar.

Additional reading

Wikipedia has good articles on Esperanto vocabulary, gender reform, profanity and more.

A compilation of terminology on sexual identity and orientation.

A guide on how to Esperantize your name.

Slang: Wikibooks, Wiktionary.

Esperanto proverbs from Wikiquote.

Other Esperanto pages in Wiktionary.