ZACH-LIKE is a book of behind-the-scenes design documents from Zachtronics. The digital version of ZACH-LIKE (which can be found on Steam or itch) contains the full contents of the book, along with a bunch of games that Zach made before SpaceChem, and some prototypes and early builds of Zachtronics games, both released and… not.
One the Internet Archive website two pages are dedicated to the unreleased GBA RPG game Remember Me:
On the original Kickstarter page for the game Wasteland 2 from inXile Entertainment (published by Deep Silver) the vision document can be found.
David Candland is really open about his UX/UI designs for Destiny and Halo which he developed at Bungie. On his website you can find HUD specifications, prototype animations, flowcharts etc. It’s really interesting and educational to see his design process.
The game pitch document for Mythos has surfaced on the internet. Mythos was a multiplayer RPG developed by the now defunct Flagship Studios.
Bob Pape, the original creator for R-Type on the ZX Spectrum released a book about the creation of the game, which can be found here
Don’t forget to browse the rest of his website, because more interesting content can be found there (sprite sheets, adverts, etc.).
The website mmorpg.com hosts the official art book for the game “The Chronicles of Spellborn”. This art book was released as a download for the US market, because it was only available in the EU as a boxed purchase.
Splash Damage released the official game design document, art book and soundtrack for the game Dirty Bomb. Originally developed as part of a Kickstarter reward, but now available for everybody.
Several game design documents and the original game proposal for Azrael’s Tear can be found on the website of Matthew Stibbe. Azrael’s Tear is a first-person adventure game released in 1996.
As part of the “Indie developer science fair”, the website Polygon released the official pitch document for The Flame in the Flood video game from the studio The Molasses Flood. This pitch was mainly used for press and investors. On the linked page the lead designer, Forrest Dowling, describes how this document was used. The other pages from Polygon about the science fair are interesting to read as well.