Most people don't choose reincarnation. Reincarnation finds their way to them. Perhaps they did something great in their life. Or maybe truck-sama wasn't looking where it was going. But without fail, those who reincarnate are destined for something great. A new life where everything changes for the better. But... Reincarnation isn't always the greatest thing. Sometimes you just draw the short end of the stick. Di Tou just happens to have drawn the shortest stick. Pulled out of his life randomly. Taken by a crazy old eccentric man not as a disciple, but as a body double?! And a janitor body double no less?! Grasping a system to find... It's a JANITOR SYSTEM!? Truly, how unlucky can one get?
If he really wanted to read through every bit of information in this outer library, even if he was parsing and flipping through each item it wasn't a task that could be completed in a few years. He still had to prioritise somewhat, especially considering he didn't plan on spending all of three years, much less all of the year or so he would stay at the Academy in the library. He still had other things to do here, as well as other avenues for information.
In other words, he had to pick and choose what to read, and what to exclude. Of course, it would still be good to read a few items from each section to get a better gist of what it contained, what style it was written in, and if or if not it had the potential to be helpful.