It's a little more tricky than that.

You'll probably want to study up on advanced math concepts before tackling this. Particularly fractals.

Usually for a fill random generation you're going to use some advanced math. It's not just one function. It's going to be a variety of passes and combinations.

I think it's a very detailed combination of fractals for the main passes which do the fills.

So... first is a 2D heightmap that can be generated by a modifed perlin noise algorithm.

With a field clamp now, they go ahead and start filling in areas. And also checking for certain rules to make sure the world looks correct before they start editing.

Then they go in and carve out rooms. From common patterns I have seen. It's a mixture of Marble, Mandelbrot, and sawtooth. They probably use an algorithm like celluar automa to check for neighbors, following some rules, and connect rooms together.

As far as dungeons and such. Those look to be prefabs.

OP: I'm not sure what your current education level is, but "Trig, algebra, calculus, discrete math, vector algebra, and algorithms" is a huge, totally non-specific list. Furthermore, you could take multiple classes in all of these (except maybe for algorithms) and still be totally clueless as to how to apply these to randomly generated worlds. Unless you're looking for the mathematics to be your primary focus, I think using the internet to find specific things will be a much better option than books.

Don't get me wrong, a good grasp of mathematics will always help, and if you want to learn all the mathematics required for a really in-depth, intuitive understanding of what's going on, then good! But you don't need to learn all of those things to be able to use an algorithm.

OP: I'm not sure what your current education level is, but "Trig, algebra, calculus, discrete math, vector algebra, and algorithms" is a huge, totally non-specific list. Furthermore, you could take multiple classes in all of these (except maybe for algorithms) and still be totally clueless as to how to apply these to randomly generated worlds. Unless you're looking for the mathematics to be your primary focus, I think using the internet to find specific things will be a much better option than books.

Don't get me wrong, a good grasp of mathematics will always help, and if you want to learn all the mathematics required for a really in-depth, intuitive understanding of what's going on, then good! But you don't need to learn all of those things to be able to use an algorithm.

The reason why I mentioned them, is that the use of fractals and graphics requires methods from all of those fields. Yes, the general idea of them is important, because everything involved in the concept of generating an entire world, is basically the use of all the general concepts of math.

And combining them into something useable is no real easy task on it's own.

Celluar Automata, which is often used to combine rooms together is a procedural algorithm that follows a set of rules. These rules can not be used correctly in a setting featuring two dimensions or three without Vector Algebra.

The vector algebra will allow you to draw shapes in the grid fairly easily, and fill out it's area in between correctly with minimum effort. The use of matrixes to rotate entire functions is also pretty important in this category.

Calculus, because you will be doing a number of derivations in a variety of fields, using falloff functions, basis, logarithms to find limits, blending, ect ect... because usually fractals are some sort of data over steps or seeds.

Discrete math more specifically because it allows you to make sure that this equation will work for all given situations, or only for a few. And if it only works for a few, it means you can be sure that you will not run into errors. This will also help you learn how to layer equations together.

Trig is the base steps that is needed in all upper levels of math involving spatial relationships.

The use of them alone won't be immediately apparent, just as learning how to code won't be immediately useful right off at the beginning. But to walk you must first crawl. That is to understand the concepts of the primitives first that drives everything else in the procedural generation process.

Mmmm... most game math books won't really focus on a language. And if it does it's usually C#. And most of what I have seen only does 3D. But it's just as easy as removing one of the dimensions from the math. Give me a few, and I can go tearing through my Universities Library, and my own collection to see if I can come up with anything.

But, the best alternative would probably to seek an actual professional class.

EDIT:

Essential Mathematics For Games & Interactive Applications by Van Verth.

This books will go over a lot of the basic concepts you will find in a Vector Algebra class, but it does an excellent job at giving examples to it's uses in practical applications. There are also a number of other math concepts that you would need to know for anything interactive, or games. Such as Collision detection, interloping, splines, ect.

Mathematics for 3D game development and Computer graphics, Third Edition.
Yes, this is a book on 3D math. But it does not mean it doesn't work for 2D math as well. As once again, it's simply a matter of removing the third dimension from the math. Perfectly doable for most applications.

Then... this website.
http://gamemath.com/

Which is more of a direct knowledge than general concepts