You may use tzedaka (ma'aser) money to buy seforim. Because the books then become public property, you must write in the books that they are ma'aser and anyone may use them. You may only do this if other people who are not in your family will also use them. 
Note You may only use tzedaka (ma'aser) money to buy seforim that are not commonly found in Jewish homes; you may not use this money to buy a siddur, chumash, or Talmud.