One is not supposed to do any activities that make a person happy during the nine days. See SA Orach Chaim סימן תקנא & Mishna Berurah quoting the Magen Avraham. Is one allowed to buy new Seforim/Judaism related books during the nine days? Would it matter if he doesn't need them for his learning now?
The Kovetz Halachos (Rav Shmuel Kamenetsky) pg.175:17 writes that it is permissible to purchase a sefer which is needed during the nine days. However, to just buy a sefer without a specific need is prohibited.