Though this is written by the linguist behind Game of Thrones’ Dothraki, this isn’t a populist cash-in type of book. It goes into the history of conlangs (constructed languages) a little bit, and then delves deep into all the ins and outs of creating a convincing one — from phonology to grammar to script. It’s fascinating, if sometimes a little hard to follow for someone who isn’t interested in building their own invented language, and thus doesn’t have something to apply the ideas to.
The book covers a lot of ground by including some case studies of invented languages as well (Dothraki, unsurprisingly, included). Less usefully for me, it includes phrasebooks for some invented languages.
Ultimately, I think you have to be pretty darn into conlangs to get much value out of this, but it is a fascinating subject.