the unfolding of language, by guy deutscher
An exploration of the ways in which languages change, including an explanation of how grammatical features (amongst other things) evolve. Utterly fascinating, and liable to change the way you think about and develop conlangs.
the search for the perfect language, by umberto eco
This is not the easiest of books to read, but well worth persevering with.
It is the history of the search for a 'perfect' language, that is, a language which could completely and unambiguously express any concept.
From the Middle Ages this took the form of a search for the supposed original language spoken by Adam in the Garden of Eden, with candidates ranging from the obvious (Hebrew) to the absurd (French!).
With the Renaissance and the rise of Humanism, this changed to attempts to invent such a language. If you believe conlanging is a new idea, think again -- there are some illustrious forerunners to modern conlanging.
There are several quite detailed accounts of some of the languages, including a chapter on the Real Character of Bishop Wilkins, which (way back in my youth) was one of the inspirations for my own interest in conlangs.
This site should be your first port of call for all things conlang. It is an on-line magazine dedicated to conlanging, and on it you will find articles, discussions, and links to other conlang sites. Don't miss it.
The home site of the Scattered Tongues web-ring, a good place to start hunting down other sites about conlanging.
Mark Rosenfeld's site. Much more than just conlanging, but this site contains his Virtual Verduria world, complete with grammar, dictionary, and tutorial for one of the languages, and plentiful information about other languages.
A new discovery -- Toki Pona is a minimalist auxlang, very simple to learn, and partly the subject of my essay on the ideal auxlang. Personally I find it flawed but intriguing, and if you have any interest in auxlangs, or in conlangs in general, do yourself a favour and visit the website.
Moving slightly away from pure conlanging, this is an excellent site if you are interested in different scripts, and has a wealth of examples and information.