Shalom u'vrachah. An author who wrote a halachic compendium or a Torah work and received approbations for it from leading poskim at the time of writing, and afterward cast off the yoke of Torah and mitzvot — is it permitted to study from his books that were written when he was observant of Torah and mitzvot? And likewise, what is the halachah regarding a STaM scribe who wrote when he was observant of Torah and mitzvot and afterward cast off the yoke?

Shalom u'vrachah 

 There is no prohibition against using what he wrote when he was righteous and presumed to be God-fearing. 

The ruling of our teacher Rabbi Amram Fried shlita. 

Based on Shulchan Aruch, Yoreh De'ah, siman 1; ibid., siman 281; and Orach Chaim, siman 39. And in Ketzot HaChoshen, siman 34, se'if katan 23. 

Have an additional question on this topic or need clarification? Leave your comment below. (Please note that the comment will not be published but will be sent directly to the answering Rabbi for review and a private response)