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.
The “Independent AAA” game developer Ninja Theory is quite open about it’s game design methods. On their website you can find the game concepts for the unreleased game Razer, including design docs and concept art. Also the combat design document and Pre-vis trailer for the game Kung Fu Story is explained throughout.
After one year of releasing the game “What Remains of Edith Finch”, Giant Sparrow has released some really interesting content about their game. On a blogpost on their website you can find a Greenlight pitch trailer, a game design document and a moodboard.
User Defaultplayer001 has put up a very interesting link to the design document for the original Deus Ex game with annotations by Warren Spector. Also a link to the Thief 4 submission document is available. Have a look on the reddit page here