I'm enjoying Book of Demons very much and am curious what the other books are going to be.

I do not think there will be any more. Maybe some other games, or maybe Book of Demons 2. Please keep in mind that tis game is strongly inspired by Diablo Series (especially diablo 1).

If you haven't played the game, why respond in the forums? They're very explicit about their plan to make 7 games in 7 genres.

The devs published a "what's next" on the dev blog at thingtrunk.com, which I'm not allowed to link to.

Highlights:

- Free patch updates to Book of Demons in 2019, including some new features.

- Book of Demons paid DLC, "more strategic mode", maybe 2019, maybe 2020.

- Book of Demons alternate prototype. This would be a reimagining of Book of Demons and could fairly be called Book of Demons 2. If they decide to go forward with it, maybe 2019, maybe never.

- Book of Aliens (second Book of the main series), "a few years away", probably 2021-2022.

- Book of Aliens definitely will feature multiplayer.

Thing Trunk says the Books in the series will be tributes to 90s games that they loved. I can't know which games they loved, but we can look at what 90s games were popular. I looked at Wikipedia's "List of best-selling PC games" and geek.com's "The Best PC Games of the 1990s".

Thing Trunk says 7 games in 7 genres, so categorized by genre:

----Adventure game (exploration/puzzles)
Return to Zork
Myst
Full Throttle
Phantasmagoria
Police Quest: SWAT
Riven
Blade Runner
Grim Fandango

----4X
Civilization II

----RPG
Baldur's Gate
Fallout
Planescape: Torment
System Shock 2

----FPS
DOOM
DOOM II: Hell on Earth
Duke Nukem 3D
Quake
Quake II
Unreal
Half-Life
Star Wars: Dark Forces
Star Wars Jedi Knight: Dark Forces II
Unreal Tournament

----RTS
Command & Conquer
Warcraft II: Tides of Darkness
Command & Conquer: Red Alert
Total Annihilation
Age of Empires
Starcraft
Command & Conquer: Tiberian Sun
Age of Empires II: The Age of Kings

----Dungeon crawler
*Diablo

----"Real-time tactics"
Commandos: Behind Enemy Lines
Hidden & Dangerous (?)

----Simulation
Theme Park
Anno 1602
Railroad Tycoon II
SimCity 3000
RollerCoaster Tycoon

----Space shooter
Star Wars: Rebel Assault
Wing Commander 3: Heart of the Tiger
Star Wars: TIE Fighter

----Racing
Grand Prix 2
POD

----Classic arcade remake
Frogger

----Hunting
Deer Hunter

---Trivia
Who Wants to be a Millionaire?

Diablo and the dungeon crawler genre have already been used in Book of Demons. 7 genres is enough to cover most of this list.

The next game is Book of Aliens. Which games have a prominent alien theme?

System Shock 2
Duke Nukem 3D
Quake (franchise)
Unreal (franchise)
Half-Life
Star Wars (franchise), kind of
Starcraft

So it looks like Book of Aliens is fairly likely to be either a first-person shooter based on Quake/Half-Life/UT or an RTS based on Starcraft, with an outside chance of an RPG based on System Shock or a space shooter. Space shooter has the advantage that "Aliens" is a really, really strong theme for a space shooter.

FPS is a really prominent category of 90s games and all of them use either an alien theme (Duke Nukem / Quake / Unreal / Half-Life / Star Wars) or a demon theme (DOOM). So if I were managing the Book of X franchise then Book of Aliens would be an FPS. Starcraft is popular and iconic enough that I think there's a good chance Book of Aliens will be Starcraft and a future Book of First-Person Shooting will just have to deal with the name issue somehow.

On a more personal note, the games that *I* loved in the 1990s were Civilization, Monkey Island, and Angband. I really hope they do a Book of Turn-Based 4X. Book of the World or Book of History or something. :D

We can also think about which games get called out in avatars, though I'm guessing those won't be the basis of future Books:

Snake (1976)
Lemmings (1991)
Frogger (1981)
Sonic franchise (1991)
Angry Birds (2009)
Donkey Kong (1994ish)
Caveman avatar -- not sure what this is
The Witcher (2007)
Minesweeper (1990)
Leisure Suit Larry (1987)
Bejeweled (2001)
Prince of Persia (1989)
Tomb Raider (1996)
Diner Owner -- ??
Frozen Ninja -- ??
Witty Mystic -- ??