Writing and buying Seforim is something lots of Jews do. There is a positive commandment of writing a Sefer Torah. This means to either write it oneself according to all the halachic requirements or paying a sofer to write it. Some say that even writing one letter is like writing the whole scroll. This is seen in the celebrations accompanying the completion of a sefer Torah where the last few letters are completed by individuals before the new Sefer Torah is escorted with a procession to it’s new home – usually in a shul.
The Rosh extends this mitzva to include buying any sefer which one will learn from. This means every time we buy a sefer we should think and know that we are doing this positive mitzva! The reasoning is that the writing of the Sefer Torah is to spread the word of Torah. Originally nothing other than the Written Torah was permitted to be written but with the passing of time and the threat to the Torah, even the Oral Torah can be written. Thus, any sefer that achieves this goal is equated to the writing of the Sefer Torah as both come for the same purpose.