Flash is actually more difficult to recommend anything because the book has been messed up for a long time. The good stuff for me is quite a ways back to the dark days of the nineties.
My start on the character was The Return of Barry Allen. Mark Waid does a great job of getting you up to speed so you won't by confused about the who's and what's and it'll probably give you a good foundation of where to hit next.http://www.amazon.com/The-Flash-Return- ... 565&sr=1-4
There are a bunch of other Mark Waid penned books that you can search out if that interests you. Otherwise, your best bet, again, is Geoff Johns. His first run on the title is pretty damn good stuff even though it also started in the nineties.http://www.amazon.com/Flash-Book-Blood- ... 797&sr=1-3
Or if you're willing to spend some change, you can get a larger chunk of story in Omnibus form.http://www.amazon.com/Flash-Geoff-Johns ... 797&sr=1-1
And if you're looking for something more modern, I guess your best start would be the Brightest Day relaunch. It doesn't last long before it gets mixed up into Flashpoint territory but it's a very good comic.http://www.amazon.com/Flash-Vol-Dastard ... 997&sr=1-4
And here's the latest run on the book. I haven't read it so I can neither recommend or burn it in effigy.http://www.amazon.com/The-Flash-Vol-Mov ... 104&sr=1-2
Not available in book form quite yet but you can probably search out the regular comics if you're interested.
And what if you want to start from the VERY beginning? And by "VERY beginning," I mean Silver Age Beginning... You can search out the black and white Showcase Presents book! It looks like this:http://www.amazon.com/Showcase-Presents ... 233&sr=1-4
This is the book that launched DC's Silver Age line of super-hero comics so it's got some historical significance. If you want to read these things in color, you can get them in Chronicle form:http://www.amazon.com/The-Flash-Chronic ... 306&sr=1-1
Oh, you can get Green Lantern that way as well.http://www.amazon.com/The-Green-Lantern ... 341&sr=1-1