[Dunan]

Upvoted; I love this book. Amazon recommended it to me out of the blue when I started studying Esperanto again and I went in totally blind; enjoyed it from start to finish.

Also, if you're interested in languages invented for TV and movies, read David Peterson's "The Art of Invented Languages", in which he describes how he created Dothraki and a whole bunch of other languages, including writing systems. As a linguist who would love to get into that kind of thing (and who has studied some very obscure languages which would make good influences), I totally enjoyed this. A little more technical than Arika Okrent's book, but still very accessible.