Can one use maaser money to buy seforim for personal use?
Maaser money should generally not be used for this personal purpose.
However, it is permitted if a person would otherwise be unable to learn from the relevant volume (meaning, the sefer is not otherwise available, and the person does not have sufficient resources to buy the book without using maaser money).
Please see sources for more details.
The most basic purpose of maaser money is for the poor.
Thus, the Maharil rules that it is forbidden to use one’s maaser money for the purpose of purchasing candles for shul. The Chasam Sofer (Yoreh De’ah 232) understands that this is because the obligation of maaser relates specifically to the poor (the Chasam Sofer writes in siman 231 that this ruling derives from the custom of giving maaser to the poor).
Yet, both the Taz (Yoreh De’ah 249:1) and the Shach (249:3) rule (citing from the Maharshal, who in turn cites from Teshuvos Rabbi Menachem) that maaser can be used for any mitzvah that a person would otherwise not perform, such as purchasing books for learning from or for lending to others.
The Maharshal (glosses to Tur 247 and 256) adds that “today we give money to charity in the knowledge that it can be used for all public needs, and even our maaser moneys are given for any mitzvah matter.”
In practice, the majority of authorities permit the allocation of maaser money for the purpose of mitzvos other than charity. Note that some contemporary Poskim write that if one begins to contribute one’s maaser money to the poor, this can constitute a neder and hatarah is required to change one’s practice (see Kovetz Teshuvos of Rav Elyashiv zt”l, Vol. 3, no. 147; see also Ahavas Chessed, Chap. 18, no. 2, where the practice of donating the money beli neder is recommended).
However, it stands to reason that because it is common practice today to give one’s maaser money to any worthy mitzvah cause, a donation to charity does not imply a neder, and it is thus permitted to give any maaser donation to an alternative mitzvah cause. This ruling is given by the sefer Orach Tzedakah (p. 369) in the name of Rav Nissim Karelitz.
It certainly remains preferable to use maaser money for mitzvos that benefit the poor, and not for mitzvos that have no bearing on the needy (see Shut Betzeil Ha-Chochmah, Vol. 4, no. 161; Minchas Yitzchak, Vol. 8, no. 83). Thus, there is room for leniency in the matter specifically where a person will have difficulty in fulfilling the mitzvah without using maaser money.