In regard to the question of Robert Tabak concerning Iggeret ha-Shemad:
Apart from the sources that were already been mentioned by Liran Yadgar – see:
1. Jacob I. Dienstag, 'Letter on Apostasy' or "Treatise on Martyrdom: A Bibliography of Editions, Translations, and Studies." Kiryat Sefer 56: 2 (1981), pp. 356-368.