sorry for more questions, they just keep on pooping into my mind.
Lol, a little spelling error , but yea its cool man, I like to do my best to help

lets say I'm reading a book in the HP Realm about History of Magic, would my brain just fill in stuff if the books didn't say anything about it?
Yes it would do exactly that. It may add stuff that would be more relevant to Harry Potter, but it could also add in completely random stuff as well. Be prepared for anything .

And if I wanted to have all the spells, could I just go on the HP wiki and read all the spells and their effects and descriptions and then my brain will remember them all?
This has been debated alot, but I believe that yes, it can remember them all. It would only make sense considering the fact that every night in your dreams, your brain creates scenery, characters, flavors in food, and all that other great stuff. In short, I would say yea, your brain definitely would. However, if maybe you wanted to make sure without a doubt that it would remember them all, you should read over those spells like you would normally read anything. I wouldn't just skim over them and read really fast(Unless you are a fast reader), but thats just what I think. Skimming over the text could maybe work as well, but I'm not sure, you'd have to try it out.