I agree with the B'run Water resource. The Water Book is also good. However, one thing I will say is this - Sometimes the technical understanding of water chemistry can be a little overwhelming for some.
The reality is this - the functional manipulation of your water to make good beer is really fairly simple and straight forward.
Honestly, you can have a functional process with as little knowledge as " Use RO water and add "X" number of tsp of CaCl and Gypsum, and add "X" ml of lactic acid" - basically, you could have 3-6 "water recipes" and get yourself in the ball park for almost any beer style.
Eventually, it is awesome and desirable to understand the details, but also know that you can start down the road to improving your water without knowing all of those details. I think too many people see the entire area of water chemistry as "too much" and abandon it entirely. Taking the first simple steps can be done with very little understanding of the entire picture.