I was led to muse just today that almost all the recent questions on lilypond are about spacing and tweaking positions of some sort or other. [Well, I suppose you could argie that engraving is only about spacing objects and nothing else anyway!] With no disrespect to any documentation developers, this does seem to be a major stumbling block for people, including me, and I am using lilypond professionally for years. I recently referred in a post to the Missing Manual for Lilypond. This topic would be a good choice for the main contents of such a doc. I'd volunteer to write but but although an experienced user, I just lack something fundamental understanding lilypond internals that leaves me permanently flummoxed. For example, when to use extra-spacing-width, which always eludes me until I post to the list.
Horizontal spacing perpetually bugs me. I need to do very strict proportional spacing to set New Complexity School scores for the composer I engrave for, and it never works satisfactorily, and seems hit and miss at the best of times. Lilypond does a beautiful job of heuristically laying things out but when you want to constrain it, for example, to make bars exactly 3cm wide, it just does not respond. I'd like to see a really good explanation of that topic, and spacing sections, which have _never_ been able to get to work.I use the latest dev. versions at all times. Currently I am hitting issues where the notes at the end of lines smash up too hard against the barline, and understanding why that is so is beyond me, let alone how you work out how to address this.
Just my A$0.02 worth.