The most useful book I have for violin is The Violin Lesson by Simon Fischer. I bought it before I knew how I was ever able to afford violin lessons because it says it is for teachers and those self-teaching the violin. I very quickly realised that with my history of tendinitis, I was not comfortable trying to teach myself.
Instead, whenever I feel down during a practice session, I take a break and read a relevant section in the book. Usually there are exercises I can work on. Eve when I don’t feel up to playing more in the moment, I come away having considered the problem from a different angle. A good number of exercises in the book are playable for beginner violinists, and I find the writing detailed and encouraging.
While some concepts (especially about tone production) are transferable to other string instruments, I wish I knew of a similar type of book for the cello. I have a copy of the book The Intermediate Cellist by Milly Stanfield, but it is a much shorter book and doesn’t have the capacity to cover so much.