Mass Effect - Timeline of Materials (minor spoilers)

As I prepare for my Mass Effect 3 playthrough, I'm painfully aware that of the myriad extracurricular materials that fill in gaps in the franchise's lore. It's unfortunate when game companies handle their intellectual properties this way; people play the games but don't comprehend some things because they didn't read Part II of some obscure tie-in comic book. Unfortunately, Mass Effect 3 suffers from this problem more than most.

There is more than what I list here - like the mobile game that was entirely focused on Jacob Taylor, and that other Mass Effect novel that wasn't written by Karpyshyn - but most of the important stuff is here. I tried to keep the list as spoiler-free as possible.

Mass Effect Evolution (comic) (four parts) (Mac Walters)

Featuring the Illusive Man, General Williams (I believe Ashley's father, though no mention is made of her), Saren Arterius, Desolas Arterius, First Contact War. 

A prequel to the prequel. Not sure why BioWare felt the need to write this series, though it does kind of explain how the Illusive Man got his bluish eyes.

Mass Effect Revelation (novel) (Drew Karpyshyn)

Featuring Saren Arterius, David Anderson, and Kahlee Sanders

Tells the story (only briefly described in the first Mass Effect game) of David Anderson's previous encounter with Saren Arterius. Also the first time we meet Kahlee Sanders. Sanders only appears in one mission in Mass Effect 3, but she is a primary protagonist in all three Karpyshyn Mass Effect novels.

Alternate place for Mass Effect Homeworlds Vol. 2 (comic) (Mac Walters)

Told as a flashback, it hints at /spoils the endings of Mass Effect and Mass Effect 2, and certain events in Mass Effect 3 in the final panels, but the majority of content occurs here. If you don't want spoilers, read it at the other spot I placed it in this timeline.

Featuring Tali Zorah vas Neema, Dr. Michel, Barla Von.

The events immediately preceding Shepard meeting Tali on the Citadel in Mass Effect 1.

Mass Effect (first video game)

Mass Effect Ascension (novel) (Drew Karpyshyn) 

Featuring Kahlee Sanders, The Illusive Man, and the Quarian flotilla, Paul Grayson

This and the next novel are very important in the Mass Effect story; pity that so few people have read them. Sanders now works at the Grissom Academy, working with specially talented children and teenagers. The Illusive Man sees the Academy as a potential source of research subjects (for the advancement of humanity as a whole, of course). One of the "subjects" he is desirous of obtaining flees to the Quarian flotilla - leading to the events and distrust of Cerberus that Tali describes near the start of Mass Effect 2 (she is not present in the novel, though).

Mass Effect Redemption (comic) (four parts) (Mac Walters) (Prequel to Mass Effect 2) 

Featuring Liara T'soni, The Illusive Man, Miranda Lawson, Feron, Collectors, Shepard's body

Details exactly what Liara's debt to Feron was. Fills some major gaps in Liara's story between the first game and ME2 Shadowbroker DLC.

Mass Effect 2 prequel: Mass Effect Incursion (comic) (Mac Walters) 

Featuring Aria t'Loak, Collectors (Incursion)

When Shepard shows up on Omega saying he's hunting Collectors, Aria doesn't seem surprised. This is the story of how she already knew about the Collectors' plans.

Mass Effect 2 (second video game)

Concurrent with Mass Effect 2:
Mass Effect: Paragon Lost (animated film) 

Featuring James Vega, Collectors (and others incl. Liara T'Soni and David Anderson, but only Vega is voiced by the same voice actor from the video games).

Vega is easy-going but conflicted when we meet him in Mass Effect 3. This is the story of the mission that cracked his confidence. 

Concurrent with Mass Effect 2:
Mass Effect Homeworlds (comic) (Mac Walters) Vol. 3. Read after the ME2 Archangel mission to avoid spoilers. Preferably, read after receiving Garrus' loyalty mission, but before completing it.

Featuring Garrus Vakarian, Sidonis, Archangel, Garrus' father, Garrus' mother. 

Some background into Garrus' family, and the story of his betrayal immediately prior to meeting Shepard in Mass Effect 2.

After the events of Mass Effect 2, but before Mass Effect 2 Arrival: 

Mass Effect Retribution (novel) (Drew Karpyshyn) 

Featuring the Illusive Man, Kahlee Sanders, David Anderson, Kai Leng, Aria T'Loak, Omega, Paul Grayson

The Illusive Man exacts revenge for the events depicted in Mass Effect Ascension. Probably the most important external material of the Mass Effect franchise. Contains a detailed, first-person account of Reaper indoctrination. Also contains Anderson's confrontation with Kai Leng that Anderson only hints at in Mass Effect 3.

After the events of Mass Effect 2, but before Mass Effect 2 Arrival: 

Mass Effect Inquisition (comic) (Mac Walters) 

Featuring Captain Bailey, Councilor Udina, Executor Pallin (Inquisition)

How Captain Bailey became head of C-sec.

Mass Effect Conviction (comic) (Mac Walters) 

(Days after the events of Mass Effect 2: Arrival DLC) 

Featuring James Vega, David Anderson, Omega

After the events of Paragon Lost, Vega starts drinking heavily. Anderson recruits him to guard Shepard after Shepard turns himself in after the events of the ME2 Arrival DLC.

Mass Effect Invasion (comic) (four parts) (Mac Walters) (prequel to Mass Effect 3: Omega DLC) 

Featuring Aria T'Loak, The Illusive Man, Omega, General Petrovsky, Adjutants

The story of how Aria T'Loak lost control of Omega to General Petrovsky

Mass Effect Homeworlds (comic) (Mac Walters) Vol. 4 

Occurs immediately prior to the opening events of Mass Effect 3.

Featuring Liara T'soni, Admiral Hackett, The Illusive Man. 

How did Liara wind up on Mars looking for Prothean blueprints in the opening of Mass Effect 3? Here's the story.

Mass Effect 3 (third video game)

Concurrent with Mass Effect 3: Mass Effect Homeworlds (comic) Vol. 1 (Mac Walters) 

Told as flashbacks, which are framed by the Reaper invasion of Mass Effect 3.

Featuring James Vega – Not sure when to place this, no real time-frame of the flash-back, other than it happens from the time before Vega was a soldier. The beginning and ending are in the early stages of Mass Effect 3, so I put it here.

Concurrent with Mass Effect 3: Mass Effect Homeworlds (comic) Vol. 2 (Mac Walters)

Told as flashbacks, which are framed by the Reaper invasion of Mass Effect 3 and the events on Rannoch. Recommend reading immediately after completing the Rannoch mission.

Featuring Tali Zorah, Chellick, Dr. Michel, Barla Von

Most of the events in this comic occur prior to or concurrent with the first Mass Effect game, but the final panels of the comic contain spoilers for Mass Effect and Mass Effect 2, so I put it here.