As someone who’s worked in book publishing as an editor, one thing books are is comparitively cheap to print - even in modest print runs.
As digital print-on-demand press begin to rival litho for sheer quality, even the need for a print run subsides.
One thing a computer book needs is lots of ‘demonstrations by example’, great editing, and a flawless index.
O’Reilly make exceptionally well-written and indexed books. Peachpit’s Visual Quickstarts are useful alt-manuals.
Some very big publishers are guilty of turning out some practically useless slop however.