This book on level design is a great resource for level artists. Since they work closely with game and level designers when building game environments, understanding the principles of level design is essential. The book covers how to guide the player's attention within a level and how to tell stories through environmental details.
It’s available for free
online, published by the author.
If you're planning to order a printed copy, go for the color edition — the section on color coding is much easier to understand in full color than in black and white.