Tim O'Reilly recently published an article about book sales and, in 
particular, gave a breakdown of units sold per programming language:

  http://radar.oreilly.com/archives/2007/05/state_of_the_co_10.html

As the author and publisher of a book on OCaml, this naturally caught my eye. 
So I spent some time dissecting the results and the conclusions were quite 
interesting:

  http://ocamlnews.blogspot.com/2007/05/ocaml-revolution.html

Most notably, we are outselling APress in terms of units sold. Also, the OCaml 
community is desperately short of cheap introductory books.

I should also note that the publication of Practical OCaml, rather than 
reducing our sales due to competition, actually increased our sales 
substantially. This indicates to me that the OCaml market is sufficiently 
small that there is no risk of competition and anything done to promote the 
language improves the situation for others in the community.

Consequently, I would like to encourage any budding authors to write books on 
OCaml and help to promote the language.

-- 
Dr Jon D Harrop, Flying Frog Consultancy Ltd.
The F#.NET Journal
http://www.ffconsultancy.com/products/fsharp_journal/?e