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.

Detala Gramatiko de Esperanto (DGE):
https://lernu.net/en/gramatiko/

Smaller version of PMEG (see above)  in English.

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.