Hey. I created an account just to post this. I have many, many mods installed, and I was having this same issue. I would finish Under Saarthal, talk to Savos, and Hitting the Books would not start. I tried startquest, resetquest, setstage, talking to Mirabelle, talking to Urag, talking to the Arch Mage again. Nothing worked. What finally did work was my loading up my current save with every mod except the mods that affected my current character (alternate start, wrath of nature, anything that added a quest or item(s) that I currently had in my journal or inventory) deactivated. I then made a separate save of that, the "Lite Mod" version. I typed "startquest mg03" into the console, and voila. After that, I saved, exited, reactivated all my mods, and continued on my merry way. I hope this helps someone out.
