Sorry if this is an ignorant question, but ignorance is what I'm trying to get rid of.
When I first started learning STa"M, the sofer I did shimush with had me practice chaseros and yeseros, and learn Kesset Hasofer and Mishna Berurah 32 (with biur halacha) and that's what Va'ad Mishmereth STa"M tested me on. Since I'm writing more lately, I feel that I should know the halachos on a deeper level than the Mishnah Berurah provides. I've gone through Hilchot Sefer Torah and Mezuzah in Y"D Mechaber/Rema and am starting נושאי כלים. Can any of the rabbonim and more knowledgeable sofrim recommend a list of seforim that are absolutely necessary to "really" know STa"M and a second tier that are I should avoid until after getting a solid grasp of the halacha through the main ones?