Hi Urs,
Since you asked for opinions I will offer mine. I do quite a bit of work with LaTeX and LilyPond, but I don't use lilypond-book for a couple of reasons.
It generates a lot of extra files and folders that create clutter if you don't direct its output to a separate folder, but when I do that, links to other files (graphics for example) in the .tex file no longer work since it's in a different folder now (I keep all linked files in the same folder as the .tex file because it's on Dropbox and the full path to it is different on different computers).
Also there is (or was?) an annoying bug whereby LilyPond doesn't take elements to the left of the staff (instrument names and such) into account in calculating the line-width of a system that occasionally cause systems to spill over in the right-hand margin. In those situations I had to use goofy workarounds to make them fit.
For those reasons I prefer to simply export images (Frescobaldi makes this very simple) and link to them as with other graphics. (Using a fixed line-width in LilyPond avoids the need to scale them or anything like that.)
The main advantage I lose is lilypond-book's ability to split a single score over page breaks when it makes for a better layout (this is possible manually, but tedious), and if you were making a book purely of scores (but including a written introduction, or opening remarks or other material of that nature) I think lilypond-book is the way to go.
Ideally, invoking LilyPond in LaTeX would be rather like using, for example, the Tikz package: the code is processed when LaTeX is run (no need to use lilypond-book first), but given LilyPond's size and complexity (compared to a package like Tikz) I'm not sure if that is possible. I know very little about programming; is it possible to get LaTeX to invoke LilyPond on a computer where it is installed? Or would it be necessary to create a package that included the whole LilyPond program?