O'Reilly does the tech community a great service by publishing their quarterly analyses of tech book sales. Yesterday, Mike Hendrickson posted part 4 of the Q1 07 analysis, which details programming languages.
The book sales statistics aren't meaningful in and of themselves, because they simultaneously overstate and understate what's actually happening within the tech world. The general opinion is that Java is important, Ruby is hot, and Fortran is deader than dead, and the book sales statistics seem to back this up. Other data, like counting job postings on the web, can lead to similar conclusions. However, this isn't the entire story -- Fortran is still an incredibly useful language in certain circles (physics and climate simulations, for example), even if the people who rely on it don't buy books or hire frequently.
Although book sales don't paint the whole story, they do provide show some interesting trends and point to questions that deserve further investigation.
In Mike's 'second tier' grouping are languages like Perl and Python, which are now shipping roughly 10,000 units per quarter. That probably deserves some additional analysis; Perl is somewhat stagnant, due to Perl 5 appearing to be in maintenance mode as Perl hackers wait (and wait, and wait) for Perl 6. Also, many recent Perl titles have been hyper-focused on specific modules that aren't widely used, or are only marginally better than the online documentation. So perhaps this indicates that Perl programmers don't buy many Perl books, or perhaps it means that Perl programmers are moving to Java/C#/Ruby to pay the mortgage. Either way, this data is inconclusive.
In the penultimate tier are languages that sell less than 1,000 units per quarter, and include Tcl, Lisp, Scheme, Lua and Haskell. Interestingly, 345 Haskell books sold in Q1 07, compared to 47 in Q1 06, an over 600% increase year-on-year. However, Mike also points out: "[t]he four Haskell titles are averaging fewer than 30 copies per month".
Again, this data is interesting, but inconclusive. Ruby titles experienced a meteoric rise a couple of years ago, when the Pragmatic Programmers released two bestsellers around Ruby. Before that, Ruby titles sold poorly; recently, they are selling over 25,000 units per quarter, and the trend is increasing.
Maybe the answer here is that Haskell is poised to make a similar meteoric rise, and leave the functional programming ghetto. Maybe the answer is that people interested in learning Haskell aren't buying expensive new books, but rather buying used copies, borrowing books, or learning from online references. Maybe the answer is that the four English-language titles on Haskell aren't delivering what the book-buying public needs, indicating that the time is right for a Pragmatic Haskell to repeat the success of Programming Ruby.
Somehow, I think the answer lies in a combination of these three possibilities.
I know that when I was going through Haskell: The Craft of Functional Programming and The Haskell School of Expression, I read through them to figure out what the compiler was trying to do with the code I was trying to write. Once I got comfortable, I didn't refer back to them at all. Instead, I read through numerous papers and online tutorials. The online docs and ghci were what I referred to most to explain what was going on. Today, I am quite comfortable lending out my Haskell books to a close friend. When I was programming in Perl for a living, I would have been much more likely to order a second copy of a Perl book (through work, of course) to lend to a friend than to let go of my only copy of a book.
I wonder if my experience is typical or aberrant, and what that means to the future prospects of selling Haskell books in 2008 or 2009...
 The same analysis points out that Practical OCaml is selling under 40 units per quarter. With so little data to analyze, I wonder if this reflects the lack of buzz around OCaml, a general lack of interest in OCaml, or a general lack of interest in this particular title...