Language references:

Plena Ilustrita Vortaro de Esperanto (PIV):
http://vortaro.net

Complete Illustrated Dictionary of Esperanto is a monolingual dictionary of Esperanto first compiled in 1970.

Plena Manlibro de Esperanta Gramatiko (PMEG):
http://bertilow.com/pmeg/detala_enhavo.html

Plena Manlibro de Esperanta Gramatiko (PMEG, English: Complete Manual of Esperanto Grammar) is a book which explains Esperanto grammar in an easy-to-learn format. It was mostly written by Bertilo Wennergren and is for ordinary Esperanto speakers who want to study Esperanto’s grammar, word construction, writing and pronunciation.

La bona lingvo:
http://bonalingvo.net

Tekstaro:
http://tekstaro.com

The Tekstaro de Esperanto (Corpus of Esperanto) is a text corpus of the Esperanto language, a large collection of very diverse texts for linguistic research on Esperanto.