Mathematical Techniques in Crystallography and Materials Science, 3rd Ed. (2004) E. Prince, Springer. DOI: 10.1007/978-3-642-18711-7
This is a short and very terse book. Not easy reading, but much beloved by a generation of mathematical crystallographers.
International Tables for Crystallography Volume C: Mathematical, Physical and
Chemical Tables, (2021) T. R. Welberry ed.
DOI: 10.1107/97809553602060000117
This is a comprehensive reference on diffraction. The original edition was edited by Ted Prince and thus has extensive material on minimization. Your library/institution may make this available on-line.
Fundamentals of Crystallography. Third Edition (2011). C. Giacovazzo, H. L. Monaco, G. Artioli, D. Viterbo, M. Milaneso, G. Ferraris, G. Gilli, P. Gilli, G. Zanotti and M. Catti. Edited by C. Giacovazzo. IUCr Texts on Crystallography No. 15, IUCr/Oxford University Press.
I have the 2nd edition, which has a nice chapter on crystallographic computing. I have not looked at the 3rd edition.