So there are two components of swimming fast: the physics of it, and the physiology end of it.
Physics, it's the easy part of the equation. Develop a model, run your model, and figure out how to maximize propulsion as the human body travels through the water. Seems like most of the swim books out there are written from the physics end of it.
But the problem is that the best way to swim from a physics standpoint isn't really the best way to get across the point from a physiology standpoint. The fastest, most propulsive stroke from a physics standpoint is butterfly. Look at backstroke and breaststroke these days: both are swam over short distances, and both are taking significant elements from butterfly these days. In back, it's the extended fly/dolphin kick at the start and turns, and for breast, it's the more fly-like movement through the water instead of a 'flat' stroke.
But in the case of butterfly, it ends up not being as fast because it does not play well with human physiology. The arm stroke has no glide phase to it. Kicking fly underwater leads to oxygen debt/quick build-up of CO2. It takes a lot more energy to swim it right than freestyle does, so for 99% of the population, freestyle is faster than fly for distances more than 25 yards.
And you don't see so much talk about the physiology end other than from an injury prevention standpoint. Getting your strokes to fit your body/body type is more art than science, and at times more than a little voodoo. Best coach I ever had, he could take a group of six swimmers, tell them to do entirely different things with their strokes, and then all six swimmers would hit the top 12 at state meet with strokes that didn't necessarily look anything like what their teammates were doing.
At the elite level, Ian Thorpe does one thing and wins Olympic Gold, Grant Hackett does another and gets his gold, and Brooke Bennett does something that would seem to be totally contrary to lessons learned from Thorpe and Hackett, and gets her Olympic Gold.
Lesson of the story: once you get the basic physics of swimming down, then it's time to start tweaking your stroke to fit the physiology end of it, because sometimes working with a stroke adapted to your own body is going to be a better solution than trying adapt your body to some sort of stroke mechanics ideal.