A short introduction to glossopoiesis
Appendix A. Bibliography and links
I highly recommend that you read J.R.R. Tolkien's paper A secret vice, published in The monsters and the critics and other essays, HarperCollins, which was the first essay on language construction and is now regarded to as a classic.
If you can read Italian, Paolo Albani and Berlinghiero Buonarroti's Aga Magéra Difúra. Dizionario delle lingue immaginarie (Bologna, Zanichelli, 1994), although incomplete, is a precious goldmine of information. The book also has a very good bibliography.
For reference in English language, Rick Harrison maintains a very good bibliography of planned languages, and David Bell has a list of linguistics books that could be of interest to language creators.
Claudio T. Gnoli's Creazione e utopia: le lingue artificiali dal Seicento a oggi (Creation and utopy: artificial languages from the 16th Century to present day) is an historical perspective of constructed languages, including a few comments on language construction and language constructors in general. An English translation should be available any time soon. The author also maintains a directory of logic languages in English.
Sarah L. Highley's Audience, uglossia, and CONLANG: inventing languages on the Internet is an interesting essay on audience and fruition of scale-model and artistic languages, and on the benefits of the Internet in this field.
Donald J. Harlow's How to build a language is an historical and critical review of several constructed languages, with a special emphasis on the construction principles and criteria adopted by the authors. More basic reviews of language creation technique, in the form of how-to directions, have been published by various language creators: among these, I suggest Mark Rosenfelder's Language construction kit, already a classic and the source of inspiration of many similar guides, and Pablo David Flores' How to create a language.
A growing directory of constructed languages is found at DMOZ, the Open Directory Project. Many other listings are maintained by language creators or conlang enthousiasts. Among these, I recommend Jeffrey Henning's Model language links, Richard Kennaway's Constructed languages list and Chris Bogart's Constructed human languages.
The best public forum for discussion on language construction and constructed languages is the CONLANG mailing list. Searchable archives and a subscription form are available. Regional mailing lists, for discussion in languages other than English, and specialized mailing lists, devoted to selected topics or even to single languages, also exist.