This year the invented language Esperanto celebrated its 125th anniversary. L L Zamenhof began the Esperanto movement when he published the Unua Libro "First Book" on July 26, 1887. Esperanto is the most successful invented international auxiliary language, with perhaps as many as 2,000,000 fluent speakers and perhaps a thousand native speakers, including international financier George Soros whose surname means "will soar" in Esperanto. Over 25,000 books in Esperanto have been published. The Scottish poet, author, translator and magazine editor William Auld wrote chiefly in Esperanto and was nominated for the Nobel Prize in Literature in 1999, 2004 and 2006, the only person ever nominated for works in Esperanto. There is an official international Esperanto Academy in charge of keeping Esperanto's structure and new vocabulary consistent with its founding principles as defined by Zamenhof and the first (1905) World Congress of Esperanto. According to this website (in Esperanto) there currently 579 Esperanto organizations in the world.
Esperanto language resources (mostly) on the Internet
The book In the Land of Invented Languages by Erika Okrent is a general survey of invented languages, including Esperanto. I found two adequate online Esperanto courses for beginners, here and here. The best English-Esperanto Esperanto-English dictionary I've found is at the Majstro website. Another good one is the Reta Vortaro. RV has definitions in Esperanto with English glosses at the end of each entry. An outstanding Esperanto-Esperanto dictionary is the Plena Ilustrita Vortaro de Esperanto. Their online dictionary (free registration required, 50 words per day limit) is extremely useful for advanced students. A list of Esperanto sexual terminology is here (sorry, all in Esperanto). The only Esperanto reference grammar I've found is all in Esperanto, the Plena Manlibro de Esperanta Gramatiko by Bertilo Wennergren. It can be viewed online or downloaded in PDF format. It's well organized, exhaustive in detail and covers non-standard grammatical and word-building affixes as well as those approved by the Esperanto Academy. A good online Esperanto bookseller is the Esperanto-USA Retbutiko. It offers textbooks, reference books, subscriptions to periodicals, original literature and translations, as well as CDs, DVDs, souvenirs and Esperanto club memberships. Many national and international Esperanto organizations have their own newsletters. Some magazines in Esperanto: La Ondo de Esperanto, Monato, Mirmekobo ("anteater"). The website La Esperanta Gazetejo has free downloadable PDFs of many Esperanto periodicals, current and out of print.