Wow, when you fall down the rabbit hole of fanfiction, you don't come back.
On a related note, would recommend fanfiction/web fiction in general. Usually not as well written as an actual book. Often still pretty good, with a lot more ridiculously awesome moments, if you keep your quality standards pretty high.
I highly recommend Worm. Also, Pact, by the same author.
Worm is very good. Pact is hard to go through IMO, but it's starting to hit the point where I'm getting more invested. The problem is that I'm caught up on it, so now I have to live with reading it serially.
Cenotaph is a good fanfic based off Worm, but you do need to read Worm first or else you'll get spoiled, badly. Copacetic is also an interesting Worm continuation, but I haven't gotten through much of it so no guarantees.
Methods of Rationality is good, but it bounces between making fun of Harry Potter and political intrigue/head games, so YMMV.
The Infinite Loops are oddly addicting. Essentially, the premise is Groundhog Day, except with crossovers allowed, changes to the story allowed, ability to carry objects between time resets and different continuities, etc. Mostly a collection of short stories, but there are a bunch of them - if a favorite series of yours has one, it's worth a shot, because there are something like 10-20 different sets running in parallel at the moment