This is an entertaining book that covers the history of invented languages back to medieval times. Okrent covers the fads and fallacies that motivated hundreds of language inventors in the 18th and 19th centuries, the many attempts to create a "universal" language in the 19th and 20th centuries (including, of course, the most famous one, Esperanto), and naturally includes chapters on Elvish and Klingon as well.
This is a book about history and pop culture, not a serious linguistics text. It's pretty comprehensive and a fairly light read, though a bit dry at times. If you have an interest in linguistics, and especially if you've ever indulged in "conlanging" yourself, you will enjoy it. Some of Okrent's personal anecdotes felt like padding to fill out the book, and the appendices in the back were definitely padding.