SDCC leads to more free comics!
Detective Comics #27 is free:
Original
Batman Day Version (features original Detective Comics #27 story (still pretty good), Brad Meltzer & Bryan Hitch "reimagining" (still pretty meh), a Scott Snyder & Sean Murphy story (REALLY good) and...something...else...it's from Meltzer and Chip Kidd and I think it really sucks (adding Meltzer's script over the original story art? Who thought that was a good idea?)
Batman Eternal #1: batman75
You have to get these from the Marvel store (link your Comixology and Marvel account if you haven't already!):
Black Widow #1 (new series): WIDOW
Guardians of the Galaxy #1 (Abnett and Lanning series): DNA
Inhuman #1: BLACKBOLT
Uncanny Avengers #1: UNITY
Free infinite comics (the #1's are on Comixology but #2's aren't even showing available):
Deadpool Gauntlet #1 & #2
Wolverine: Japan's Most Wanted #1 & #2
Iron Man: Fatal Frontier #1 & #2
Link
Guardians of the Galaxy Infinite #1: Drax
Guardians of the Galaxy Infinite #2: Rocket Raccoon
And all of that Valiant stuff is still free but I expect the "a limited time" to end after SDCC. Free Digital Exclusive #1s: Rai, Eternal Warrior, Bloodshot, Shadowman, Unity, Harbinger, X-O Manowar, Archer & Armstrong, Quantum and Woody (new series and original) and the last couple years of FCBD issues. Rai wasn't bad and Quantum and Woody is pretty funny, haven't checked out the rest yet.
That looks like all for now...
The sequel (Master of the Future) is ok but not great. Gotham By Gaslight was dark Master is bright (different artist, or a better way to state that: not Mike Mignola). The story is decent until the Master plan is revealed. The story feels like a different writer trying to capture the same idea of a story without having read the story first. It's been a few years since I read the story but I remember being surprised Brian Augustyn wrote the story.