Books on Torah Study
Before listing the sources from the Rishonim concerning the goal of Torah study, I would like to note the two wonderful books that present these sources: Torah Study by R. Dr. Yehudah Levi, and Torah LiShmah: Torah for Torah's sake in the works of Rabbi Hayyim of Volozhin and his Contemporaries, by R. Dr. Norman Lamm.
R. Dr. Levy's Torah Study was reportedly put in cherem by Rav Schach for its sections about the importance of pursuing a livelihood. It also has an especially interesting haskamah from the Gerrer Rebbe. You can look at a sample of the book and purchase it here.
R. Dr. Lamm's Torah LiShmah has a different focus and is a more scholarly and detailed work. It is not just a study of R. Chaim of Volozhin's approach on its own, but rather discusses it in light of the approaches that preceded and surrounded it. You can look at a sample of the book here, but unfortunately it is out of print.
(Note: I would suggest buying the books from the publishers - Feldheim, Ktav - rather than from Amazon. Amazon forces the publisher to let them take a huge cut.)