How do professors change research interests?
"All of the above"... but maybe not wanting a book that started from a primer on Linear Algebra. (Probably the latter remark in the question is a result of a misunderstanding of some sort.) In short, all possible sources are used. I have little interest in connections to textbooks (which typically feel an obligation to include busywork exercises, etc. and express reverence to the founders of the subject by recapitulating "troubles of the old times" (which have been solved, often).)

The operational point is that one skims all these resources, looking for key points, rather than reading ploddingly, slavishly. I look to be persuaded... or, equally interestingly, to be persuaded that the people doing the thing don't adequately understand it so as to be able to explain it simply. Quasi-ironically, the latter affords more possibilities for making genuine progress. :)