For a man who has written a Buffy the Vampire Slayer lexicon, Michael Adams puts together a pretty academic volume. He and his fellow-contributors range from the Bible to Esperanto to alien-speak in computer games. JRR Tolkien invented more languages than most polyglots can speak, crafting several types of Elvish, not to mention dwarf-talk and Mordor-mumble. To him, inventing a language, and the history of the folk who would have spoken it, was an art form.
Klingon was born more prosaically. As Trekkie readers will know, one of the Klingons had a few largely inaudible lines in the first Star Trek film. These humanoids were more loquacious in Star Trek III, thanks to the inventiveness of Marc Okrand, an interplanetary linguist. One of the contributors to From Elvish to Klingon, he also wrote a book of its vocabulary and grammar. Klingon is thought to be the most widely spoken of "fictional" languages.
Of the thousand invented languages Adams has totted up, many were conceived with rather more serious aims. If everyone could speak Esperanto as their second tongue, the world would be a friendlier place (Planet Vulcan too). So thought its creator, Lazarus Zamenhof. There has been no shortage of rivals for linguistic world domination.
One of the earliest "international auxiliary languages" popped up in 1661 when an Oxford schoolteacher worked out a scheme by which the actual letters in a word would classify its meaning: anything with a "k" signified politics. A harmonious 1817 "interlanguage" named Solrésol used the seven notes of the musical scale. A German priest dreamed up the ambitious Volapük, "a universal language for all educated people". In 1888 there were perhaps a million Volapükists. Sadly, a feud caused the movement to collapse. Some former adherents invented their own versions instead: Spelin, Dil, Balta and the perhaps unfortunately named Bolak.
Polari is a borderline case. With its deliberately obscure vocabulary, this was an "anti-language" used by gays in the last century. Modern Hebrew was also used to bind its speakers together, not the world in general. The intriguing last chapter by Suzanne Romaine shows that a language could be invented, or reinvented, for reasons of nationalism (in the case of Cornish, regionalism). When Modern Hebrew was launched, its 100,000 words swamped the 30,000 mustered by Biblical and Rabbinical Hebrew. Nothing wrong with that: the first language was invented. According to Genesis, God brought the animals to Adam for him to name. "I'll call that one 'kat'. No, sounds a bit too Klingon: make that 'cat'."