Mike Hendrickson from O'Reilly has published a survey of the technical book market. As when viewing the TIOBE site, I think it is wrong to extrapolate any broad conclusions about the development languages market from the data you see here. For instance, lots of developers are still writing batch files, it just so happens that the book-market for that syntax is small. Changes in the C/C++ market are slow, which means that developers stick with tried and true books for those very popular languages rather than rushing out to buy new texts. Nevertheless, it is interesting to at least view this data, especially if you happen to be interested in technical books or trends in the broader developer landscape.