Motivation and Demographics links:
ESA Computer/Video Game Industry Facts    (and the 2016 ESA Essential Video Game Industry Facts document)  
Players Who Suit MUDs by Richard Bartle    (presenting the 4 player types)   
Virtual Worlds: Why People Play by Richard Bartle    (presenting the 8 player types)   

Storylines and Narrative in Games:
Level 10: Nonlinear Storytelling  

Sound Clips & Music:
Sound Effects @
Free Sound Effects @

Rogue-like Games (i.e. RPGs with text graphics):
Stone Soup's Dungeon Crawl      (download)
NetHack      (link to download)
Omega (for Windows)      (lvl 10 pally/priest)
DoomRL      (link to download)