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.
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
The official game design document for Sam & Max: Hit the Road was posted online on twitter by @samandmaxUK .The people behind this also run a website devoted to the Sam & Max games, which is a real goldmine about the series.
The Playstation extended play website has a great background story about the pitch for Guacamelee. Drinkbox studio’s artist Augusto Quijano gives some insight about his initial idea’s for the game and how the game was developed from then on.
Captain Claw is a 2-D sidescrolling platformer, developed by Monolith Productions for Microsoft Windows PC’s. Along with a review, the original Game Design Document (scroll to the bottom of the page) can be found here.