The Rema paskens (OC 551:2) that the issur of building and planting in the three weeks and even during the nine days does not apply if it is for a Mitzva. The Mishna Berura (14) says that if someone does not yet have children and is scheduled to get married after Tisha B'Av, they may buy clothing for the wedding even in the nine days.
The Piskei Teshuvos (10) says that based on this halocha, you may buy seforim in the nine days if you need the sefer to learn from. However, he mentions that the Steipler Gaon (Orchos Rabbeinu) was machmir not to allow new seforim into his home in the three weeks.
One time, when a box of seforim arrived during the nine days, he instructed his family members not to open it until after Tisha B'Av. So great was his simcha in learning new seforim that he was makpid for himself, since for him it was a much too joyous event.
http://revach.net/halacha/tshuvos/Steipler-Gaon-Buying-Seforim-In-The-Three-Weeks/4631
