The Guile reference manual [1] contains a good introduction to the language in general as well as some more in-depth information for working with guile specifically.
One of the most useful resources for me in grasping the important concepts of scheme was SICP, which is available in text online [2] and as a series of lectures [3]. These are quite general-purpose and not related to lilypond, but if you want to learn scheme they should be useful. [1] https://www.gnu.org/software/guile/manual/guile.html [2] https://mitpress.mit.edu/sites/default/files/sicp/index.html [3] https://www.youtube.com/playlist?list=PLE18841CABEA24090
