Tangent started as a series of one-shots in 1997, followed by a second series in 1998. Finally, a 12 issue maxi-series was published in 2008 to cap it off. It is possible to bind all of the comics into one volume, although some would prefer to keep the later series separate from the first two.
Phase One: 1997
All of the one-shots show this same order in the back, and most of them are self-contained.
Phase 2: 1998
Tales of the Green Lantern
The Trials of the Flash
Nightwing: Night Force
The Joker’s Wild
Figuring out the order of these one-shots was a little bit trickier, as there was some plot points that were shared and referenced between the different books. On top of that, there was no consistent list in any of the books. This is not the reading order presented in the official trades, or even floating around much of the internet. Thanks to canaryfarmer at the Uncollected Editions boards for his mapping, and explanations, which I have adapted here:
-Powergirl takes place in an unspecified time early on, but it absolutely takes place sometime before Night Force (the issue is referenced in NW: NF).
-Wonder Woman doesn’t show any effects from the main event in Nightwing: Night Force, so we’re assuming it takes place beforehand.
-Trials of the Flash ends just before Nightwing: Night Force, complete with editor’s note.
-Nightwing: Night Force contains the most pivotal event of Phase 2, causing power to go out across the world.
-The Superman and The Joker’s Wild both show the power going out during the issue.
-The Batman picks up after the power has been off for a while. (The editor’s note in the issue does refer to Nightwing: Night Force, and not the Nightwing one-shot from 1997.)
-JLA clearly is the final book of the event.
-Tales of the Green Lantern can go anywhere. I have chosen to place it at the beginning of this phase, but canaryfarmer placed it just before Nightwing: Night Force, where the main “narrative” kicks in.
Justice League of America #16
Tangent: Superman’s Reign #1-12
Justice League of America #16 also has a backup feature that is not related to Tangent. It does share the final page, so a scan-and-print is necessary. “History Lesson” is a backup feature from Superman’s Reign #1-11. The trades put it in the front of the books, but this page presents a case for leaving them after the main narrative. Ion #9-10 also has stories that tie into Tangent, and would go right before JLA #16, but they aren’t necessary at all.