Novelisations

I've split the list by original language to avoid confusion since the earlier games have distinct novelisations in both languages written by different authors. Official or fan translations have been noted and linked to.
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Japanese Novelisations

Metal Gear Solid 4: Guns of the Patriots
Project Itoh | June 2008
Original | Official Translation
A much-lauded piece of work from sci-fi author Project Itoh - a close friend of Kojima's and a great admirer of his work. Itoh passed away from cancer a year after this novel's release and Peace Walker is dedicated to his memory.
A highly recommended piece of work that really drives home the emotions of Guns of the Patriots.

Metal Gear Solid 3: Snake Eater
Satoshi Hase | January 2014
Original | Fan Translation
Satoshi Hase's only contribution to the MGS novelisation collection but a valuable one, notable for expanding upon the backgrounds of the Cobra Unit and elaborating on The Boss and Ocelot's relationship.
Many thanks to fancyshooting, laezels and hylaversicolor on Tumblr for the translation.

Metal Gear Solid: Peace Walker
Hitori Nojima | March 2014
Original | Fan Translation
The first of Hitori Nojima (Death Stranding co-writer Kenji Yano)'s excellent novelisations, delving deeper into Snake's psyche and giving some valuable backstory to the supporting characters. Nojima also has a great skill for enriching his novels with various historical and literary anecdotes that give all his works a very Kojima feel.
Thanks to bigmonsteras for this translation - the first full fan translation of any of the Metal Gear novels as far as I'm aware.

Metal Gear Solid Substance I: Shadow Moses
Hitori Nojima | August 2015
Original
The first of Nojima's Substance books, essentially combining Metal Gear Solid and Metal Gear Solid 2 into one complete story. I've only just begun digging into these and will hopefully emerge with an edited MTL for them soon, unless someone else beats me to it.
So far I've noticed plenty of Phantom Pain references, and a unique framing from the perspective of a new character looking into the events of the Shadow Moses Incident as a result of Nastasha's book.

Metal Gear Solid Substance II: Manhattan
Hitori Nojima | September 2015
Original
The above applies to this novel too, with this being the half of the pair I'm the most interested to delve into.
A quick skim suggests this novel really highlights the benefits of revisiting these initial games after the saga's end, with Nojima bridging the gaps with new information pertaining to characters and events established after MGS2's release.

Metal Gear Solid V: The Phantom Pain
Hitori Nojima | October 2015
Original | Fan Translation
Nojima's final novel, brilliantly retelling the events of The Phantom Pain and bringing its themes to the forefront,
with some particularly excellent character work regarding Quiet, Eli and new character Leonard.
This is my own fan translation, and while it's not perfect it was a real labour of love. I can't recommend checking it out enough, the author's take
on this game is incredible.
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English Novelisations

Metal Gear
Alexander Frost | July 1990
lol good luck tracking this down
I don't think I need to tell you a children's Metal Gear book from 1990 is going to be bad.
This is from the Worlds of Power series of NES novelisations, which was a very hilarious effort to release books about games full of violence while attempting to censoring every act of violence within them. Note Snake's gun being edited out of his hands on the cover.
Solid Snake's real name in this one is Justin Halley, by the way.

Metal Gear Solid
Raymond Benson | May 2008
Original
I'm inherently dubious of English novelisations of Japanese media and from what people say about this one it seems like that's justified.
Most discussions of this book focus on the quippy nature of Snake in it, other than that it's seemingly quite a plain retelling of the game. There is some more exploration of Les Enfants Terribles, even including a scene with Dr. Clark overseeing the birth of the twins, making this the only media to ever "officially" portray it.

Metal Gear Solid 2: Sons of Liberty
Raymond Benson | November 2009
Original
Another very straightforward adaptation, this one is talked about even less than the Metal Gear Solid one so interpret that as you will.