The article makes the book sound interesting and I'll be looking out for it, but the subsequent comments are also worth a read.
There's plenty of informed discussion of the variety of languages that have been invented and the motives of the people who invented them. Brian Barker's comments there relate to Esperanto.
One topic I didn't see mentioned was the Baha'i religion's attitude to a universal auxiliary language. My neighbor recently gave me a book about this religion, which I had never previously heard of (not proselytising, as she's not a member of that religion).
They seem an optimistic group (not unrealistically so, I hope) and aim for world unity and peace in the future.
Their site says, in relation to their vision for a future United Nations:
4. Making a commitment to a universal auxiliary language and a common script
The United Nations, which currently uses six official languages, would derive substantial benefit from either choosing a single existing language or creating a new one to be used as an auxiliary language in all its fora. Such a step has long been advocated by many groups, from the Esperantists to the Bahá'í International Community itself.18 In addition to saving money and simplifying bureaucratic procedures, such a move would go far toward promoting a spirit of unity.
We propose the appointment of a high-level Commission, with members from various regions and drawn from relevant fields, including linguistics, economics, the social sciences, education and the media, to begin careful study on the matter of an international auxiliary language and the adoption of a common script.
We foresee that eventually, the world cannot but adopt a single, universally agreed-upon auxiliary language and script to be taught in schools worldwide, as a supplement to the language or languages of each country. The objective would be to facilitate the transition to a global society through better communication among nations, reduction of administrative costs for businesses, governments and others involved in global enterprise, and a general fostering of more cordial relations between all members of the human family.19
This proposal should be read narrowly. It does not in any way envision the decline of any living language or culture.
The Baha'i site says:The Bahá'í community, comprising members of the Bahá'í Faith from all over the globe, now numbers some five million souls. They represent 2,112 ethnic and tribal groups and live in over 116,000 localities in 188 independent countries and 45 dependent territories or overseas departments. What was once regarded by some as an obscure, tiny sect is now recognized by the Encyclopedia Britannica as the second-most widely spread independent religion in the world, after Christianity.
If you put this number together with the Encyclopaedia Britannicas estimation that
more than 100,000 persons worldwide use the language [Esperanto], and several dozen periodicals are published in Esperanto(*1)then perhaps in the future everyone might have a second language that makes understanding between individuals and groups more of a possibility.
Reference:"Esperanto." Britannica Student Library. Encyclopaedia Britannica 2008 Ultimate Reference Suite. Chicago: Encyclopaedia Britannica.