Can one learn mussar on tisha b'av, or how about Kuzari or Moreh Nevuchim? Is it considered to be the study of Torah, which gladdens, or is it, since it is mussar, okay?
You are not beholden of course, but I asked this to my Rosh Yeshiva and he said mussar is definitely in the spirit of the day, but most mussar sfarim constantly quote psukim which we may not read, so the answer was no. I didn't ask him about philosophical works.
The Shaar Hakollel (R' Avrohom Dovid Lavut) writes that he heard from R' Hillel Paritcher that one is allowed to learn Kabbalah on Tisha BeAv, as one is allowed to give over the secrets of the Torah only to one who's heart is worried.
The Netey Gabriel in Hilchot Ben Hametzarim vo. 2 75:11 explains that reading\learning Mussar is permitted on the ninth of Av since it breaks and arouses a persons heart. It would seem however that philosophical works like the Kuzari and the guide for the perplexed is forbidden since reasons given for permiting learning certain parts of the Torah don't seem to apply to those types of Seforim.