Here's three very good tutorials I think everyone should read.

Gameplay (http://book.hourences.com/bookgameplay.htm)
Lighting (http://book.hourences.com/booklighting.htm)
Textures (http://book.hourences.com/tutorialstextures.htm)

This guy made several of the official maps for Unreal Tournament. These tutorials are based mainly around UT and deathmatch styles, however they are written much more generalised and I learnt much that can help me in DOD:S mapping.

I think a lot of mappers could really use conceptual stuff like this. good find.

I think is a book on level design -- he posts a lot on mapcore..good design site

Good find Ca-Chicken-Soup.
Here are some other finds about gameplay.

Bens Small Bible of Realistic Multiplayer Level Design (www.crymod.com/uploads/tutorials/Crytek/Bens_Small_Bible_of_Realistic_Multiplayer_Level_De sign.pdf)
a small bible about multiplayer level design, by Benjamin Bauer from Crytek. It shows aspects of pefect gameplay and design optimize for your multiplayer map.

And here is one more. You need to registrate :-(
An Architect's Perspective On Level Design Pre-Production (http://www.gamasutra.com/features/20030603/licht_01.shtml)

Here is another good article on level layout planning, though it is in German.
http://www.thewall.de/content/media/allgemeines_gamedesign/artikel/spielhandeln_im_deathmatch/spielhandeln.pdf

Every mapper that wants to get serious about it needs to buy Hourences' book. It's a must have