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.
My little Esperanto grammar
http://esperanto-gramatiko.saluton.dk/esperanto-gramatiko-superrigardo/English/
This booklet provides a thorough introduction to the grammar of Esperanto in 30 A5 pages. It covers all the information to take learners up to advanced level.