The popularity of superhero comics fizzled somewhat following World War II, and the Golden Age of the comics was over by the early '50s. However, by the mid-'50s, a couple of enterprising editors decided to revamp the superhero concept by adding in various sci-fi elements for the atomic age.
The book widely considered to have kicked off this new age—the Silver Age of Comics—was Showcase #4, the first appearance of the second Flash, Barry Allen. This version of the Flash got hit by some chemicals that got struck by lightning at his job as a police scientist (aka CSI) and decided to wear one of the like top five best superhero costumes of all time. This is the version we will see on the TV show (uh, with some caveats).
The Flash stories of the '50s and '60s were primarily created by John Broome and Carmine Infantino, who co-created this version of the character with Robert Kanigher, and while the early stories from this era can often be highly formulaic—even in comparison to other comics of the time—they introduce many of the key elements of the Flash mythos: the Rogues, Gorilla Grodd, Iris West, Wally West aka Kid Flash, the stretchy detective Elongated Man, and so on.
Furthermore, it is within the pages of the Flash that DC introduces the concept of its multiverse; the idea of Earth-2 being a plane of existence where all the Golden Age heroes lived and fought begins with Barry meeting up with Jay thanks to some savvy vibrating. This is a concept that has been at the center of most of DC's major even storylines, up to and including the currently running Multiversity.
Plus, you eventually get awesome stuff like this:
Boy, I cannot wait for that episode of the TV show.
If the silly inventiveness of the Silver Age is not to your taste, well, that’s your own cross to bear, I guess, but if you want to see the real roots of the modern Flash, there are a couple of ways to get these stories:
The Flash Omnibus collects the earliest adventures of the Flash from Showcase into his own title. This volume collects SHOWCASE #4, 8, 13 and 14 and THE FLASH #105-132 (the series resumed the numbering of the original Golden Age series) in color, in a big, fat hardcover. Now, even with Amazon’s pretty deece discount here, you may not want to drop sixty-plus bones on the Flash. In that case, you can get:
Showcase Presents: The Flash. This series of volumes contains the same material as the omnibus, but in black and white, on lower quality paper, but for way less money. While the omnibus (so far) only collects the first 30ish issues of Barry Allen’s adventures, there are four Showcase volumes of Silver Age Flash available (not technically in print as far as I can tell, but they’re all still available for decent prices on Amazon), which will altogether get you about 80-90 issues. These books are, in my estimation, the best value. But! If you really want to see this stuff in color, but don’t want to buy the Omnibus…
The Flash Chronicles collects, once again, the same material, but in smaller chunks than the Showcases, but in color.
Additionally, a select handful of issues of the Silver Age Flash are available on Comixology for mere pennies.
You have options, is what I’m saying.