New Seforim in the Three Weeks
Children & shidduchimTorah learning
The Rema rules (Orach Chaim 551:2) that building or planting for the sake of a mitzvah is permitted in the Three Weeks, and even in the Nine Days. The Mishnah Berurah (14) adds that a person who has no children yet and is to marry after Tisha B'Av may buy his wedding clothes even during the Nine Days.
On this basis, the Piskei Teshuvos (10) allows a person to buy seforim in the Nine Days if he needs them in order to learn. He notes, however, citing Orchos Rabbeinu, that the Steipler was stringent and would not let new seforim into his home during the Three Weeks.
Once, when a delivery of seforim came in the Nine Days, he told his family to leave it closed until after Tisha B'Av. New seforim brought him such happiness that he held back for himself: to him, they were far too joyous for those days.