Could you add a bit more info here, knowing all kind of math stuff is always useful, but I think that learning math with some specific context is the best thing to do?
What are you trying to accomplish??

There is also the esoteric area of category theory, that gets real abstract real fast but to highlight how useful the language Haskell and its entire type system is based on this.

There is no must learn maths, but the more you know and understand the more potential ways you have to solve problems :)

Never stop learning

Essential Mathematics for Games $ Interactive applications by James M. Van Verth and Lars M. Bishop

The book is well written, provides plenty of examples, figures, and diagrams and takes the time to explain them.

The book is fairly good at getting straight to the point and avoiding "terrible academia explanations"

Basic understanding of algebra, calculus, and trig is needed.

http://www.amazon.com/Essential-Mathematics-Games-Interactive-Applications/dp/0123742978

"I am trying to make a open world multiplayer game."

Well this isn't specific enough. If you wan't to make a game of any kind you 1st must break it down in separate blocks of task. if you cant break it down to blocks that you are familiar with this basically means that you are not ready to make that kind of a game.

So a minor hit is to take few techincal details that are usually needed in "open world multiplayer game" and implement those thing. For example if you have spells in your game, you might wanna know if the "opponent" is in the range of that spell, which in terms of math the question is "how do i retrieve the distance between 2 thing".

So this is basicall what I'm asing for... A specific math/gamedev problem not a general "activity"