Too long.
It takes more work, but try to make it shorter.
The best value I've gotten from books is being able to avoid the slew of occasionally useful factoids and be told what stuff I should really care about.
The endless details I can find on the Web if I need them; the problem a tech author should solve is in helping the reader know what to care about and when.
That is to say, if you're going to write something, every word has to have potency. This gets at the cut in half, cut in half, cut in half advice.
In short, good writing is concise.
A friend then said he would be happy to pay even twice as much if the author could have made it half-again as short.
BTW, I could have sworn that quote was from Chekov. Either way, on the money.
When I was one of two co-authors of a book I don't recall being told to hit a specific page count, but there was a push for a certain degree of "thoroughness." This typically meant going into possibly tangential details to avoid sending someone off to another resource. (Though if the other resources was another Wrox book then allowances we made.)
For example, in a book about XML (Pro VB 6 & XML[0]) there was a need to go through the XML specification. A whole chapter. I ended up using this as an opportunity to explain the spec and build some helper libraries that would make interacting with a DOM easier later on, so not a total loss, but it had a feeling of busy-work to me.
That was sort of a best-case scenario; two authors splitting book. I also worked on assorted individual chapters for other books and there I had an incentive to bolster the word count because in some cases it affected the pay.
In my experience writing a single chapter meant either a flat amount or a per-word rate. Writing multiple chapters opened up the option of royalties instead. I took royalties on the VB book and did OK. Sadly this gave me a false impression of tech writing as a profession.
Many Wrox books were more a collection of padded chapters than coherent, well-planned, guides. OTOH I think many people plunked down $40-50 just to get specific chapters that would get them past some problem.
[0]: As the Velvet Underground sang, "Those were different times." :)