How to unlock more master levels in Doom Eternal

The book titles refer to other games that came out during the 1990's as well as franchises that are owned by Bethesda.

1. Daikatana - Superfly Johnson is a sidekick to Daikatana's protagonist, Hiro Miyamoto, and accompanies you throughout much of the game. Daikatana was directed by John Romero, one of the major figures in the development of the early id software games including DOOM I & II.
2. Duke Nukem - The name "Dork Norkem" is a play on words of Duke Nukem. The unflattering name of the author may refer to the rivalry between id Software and 3D Realms (the developers of Duke Nukem).
3. Devil Daggers - an indie game insipired by Doom/Quake.
4. Unreal - the Liandri Mining Corporation is the organiser of the Tournaments in the Unreal setting and acts as an antagonist in many of the games.
5. System Shock 2 - the Von Braun is the spaceship in which SS2 is set, and the primary antagonist of the series is the rogue AI SHODAN.
6. Half-Life - Unforeseen Consequences is the third chapter of the game, the location where these consequences originate is the Black Mesa Research Facility.
7. Deus Ex - UNATCO (United Nations Anti-Terrorist Coalition) is the agency that the protagonist, JC Denton, initially works for. In dialogue the name is always pronounced "u-nat-co" as a word rather than initialised. The game also has a scene which became a minor meme involving the discovery of a bomb.
8. Wolfenstein 3D: Spear of Destiny - A prequel to the game Wolfenstein 3D, both of which were developed by Id Software, who also developed the original DOOM.
9. Rage - A game by id Software in the similar vein to the Fallout series.
10. Quake series - Stroggs are alien invaders that are bad guys in Quake 2 and Quake 4 and are playable characters in multiplayer games (e.g. in Q3 Arena). Their logo is also present in original Quake on so called Strogg Crates.
11. DOOM, original flavor - Flynn Taggart is a known alias of the original Doom Guy, primarily in the novelizations of the original games which... deviate from the game canon, to put it mildly, hence the mention of retconning.
12. Commander Keen - Dopefish is a large green cartoony fish featured in the game.
13. DOOM - Tei Tenga is a planet or a moon which is mentioned as off-world mining place in original Doom and in 2016 Doom reboot.
14. The Ultimate Doom - Thy Flesh Consumed is the final episode in the game.
15. Prey - The title here refers to the on/off nature of the cancelled sequel to the original Prey. It might also refer to the minor controvesy about if the 2017 game is a reboot or a sequel.
16. Fallout - The protagonists in the Fallout series are called Vault Dwellers, and an item in various Fallout games is the 'Wasteland Survival Guide'. This guide would be useful to them to survive the nuclear wasteland.
17. Fallout - Nuka-Cola is a beverage in the Fallout universe.

There is a menu option to show performance metrics in the top-right; while in-game, press Esc and go to Settings → Video → Performance Metrics. You can set it to different levels (e.g. Low to just show the frames per second or Ultra-Nightmare to show everything).