And also, I'm looking for flash comics reading order (I guess also post crisis) But i can't seem to find it anywhere...
Understanding Flash is the key to understanding his whole thing. There have been 4 Flashes. The first Flash, Jay Garrick was from the Golden Age of comics.He's the one with the bowl on his head. He was created shortly after Superman and Batman in the 1930s when super hero comics first took off. But after a while, super hero comics went out of style and Flash, Green Lantern, and other superhero comics got cancelled. Only Batman and Superman really survived.
Then in the 50s, DC revamped many of their old heroes and we got the newer more familiar Green Lantern, Flash, etc and the Justice League. This new Flash was Barry Allen (the current Flash). He was Flash for a long time. And in the 60s, Barry's nephew Wally West became the Kid Flash and joined the Teen Titans. Also in the 60s, Barry was able to travel between dimensions and he met Jay Garrick, establishing the DC multiverse. All those old forgotten heroes lived in a different universe. They crossed over every once in a while in what DC called "crises."
Then in the 80s, DC decided their 50 year continuity had gotten too convoluted and they wanted to streamline it. They wrote Crises on infinite Earths which destroyed the multiverse and merged all the interesting characters from different universes into one universe. In the new continuity, the old heroes were now from the same world as the newer ones and were the older generation (this is THE crisis that the timeline you were looking at refers to). Also, in Crisis on Infinite Earths, Barry Allen dies saving everyone and Wally West takes up the mantle of the Flash. This crisis basically served to make Batman darker and to erase any of the old campy batman from the past. It also changed Jason Todd's story, the falling out between Batman and Dick Grayson, and Batman's early years significantly.
Wally was the Flash for a long time. If you want to read all the post-crisis Flash stuff, start with Flash vol 2 #1. There are stories in there explaining the new back story, so you won't really miss anything. In the 90s, it's established that before Barry died, he and his wife Iris went to the distant future and had kids. At some point, Barry's grandson Bart Allen comes back from the future and joins Wally as the hero Impulse. Jay and Joan Garrick raise him. There's an Impulse series that occasionally crosses over with Wally's series. Flash Volume 2 earn until issue 230 before the next crisis.
Then in the 2000s, there was a second major crisis called Infinite Crisis. It rebooted things again, but in a less drastic way. It basically recreated the multiverse with 52 worlds (not infinite like before). After this crisis all the DC books pick up one year later and at this time, Bart has become artificially aged and has taken over as the Flash. His series "Flash:The Fastest Man Alive" ran for 13 issues before he got de-aged and Wally came back. Wally's series continued where it left off with issue 231. There is a single issue called "All-Flash" that bridges the gap between the two series. I don't think infinite Crisis had a huge impact on a Batman's continuity.
A few years later came Final Crisis (which is important to Grant Morrison's Batman stuff). During this story Barry comes back to life. Following final Crisis there is a mini called Flash:Rebrith which deals with Barry's return to life. But it makes almost no sense if you haven't read all of Wally's stories. If you have read them, it's pretty incredible. Then Flash vol 3 went on for 12 issues with multiple flashes. But then Flashpoint reset everything once again, leading to Flash vol 4. I haven't read any Flash since Flashpoint, but last I heard Wally and Bart had been erased from continuity and Jay was once again from a separate universe.
Grant Morrison's Batman kind of throws a monkey wrench in the whole deal because it references old weird Batman stories that were supposedly erased from continuity by Crisis on Infinite Earths. But it does it really well, so it doesn't really matter. Flashpoint changes some details of Batman's history too. The new continuity is shorter than the old, with the heroes only having been around a few years. So Tim Drake (the third Robin) was never Robin in the post-Flashpoint continuity. I guess he was still around, but as Red Robin sort of doing his own thing maybe. I'm sure there are plenty of other differences, but I'm more of a Marvel guy myself. Most of the DC stuff I enjoyed (Flash, older Teen Titans) got completely changed/erased after Flashpoint so I gave up on DC altogether.
Hope that helps!
