http://revach.net/article.php?id=4631

Section: Halacha   Category: Tshuvos
Steipler Gaon - Buying Seforim In The Three Weeks
The Rema paskens (OC 551:2) that the issur of building and planting in the three weeks and even nine days does not apply if it is for a Mitzva.  The Mishna Brura (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 Tshuvos (10) says that based on this you may buy seforim in the nine days if you need the sefer to learn from.  However he brings 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. 

Important Note: We bring this tshuvah as a starting point for discussion and not to convey any halacha.  We try to convey the Tshuva to the best of our ability. We admit that our understanding may not be accurate. One should learn the tshuva to verify the accuracy of our interpretation.  Please understand that this Tshuva may not be the final word on this topic. One should consult a Rav before drawing any conclusions.