Description has always been relatively easy for me. I think this comes from a number of the books I read in high school from writers such as Mervyn Peake, E. R. Eddison, Lord Dunsany, and others. There was so little fantasy available back then that I never questioned whether a book was difficult to read.
One of the things that did throw me as a result of my reading such brilliant writers was a tendency to write too much description, or description for its own sake, which is the way I mistakenly perceived their work, when in actuality they were almost always using description as a means to move the story forward. Once I began using description with purpose, it became more effective.
One simple guideline is to avoid ly words, modifiers which can often be replaced by a better word. "He yelled thunderously" can be replaced by "He thundered," or "He roared." Look for ly words in your writing as you go through your second draft, and try to replace them. As with any rule, this one can be broken if you can't find a better way, or if it does precisely what you want it to. (J. K. Rowling uses them quite often, with no apparent harm.)
After I've written a first draft and begin rewrites, I'm always thinking about condensing sentences, finding more efficient, less wordy ways to say things, tightening the writing up without losing the meaning. But my ultimate motto is always write for clarity. If the writing isn't clear, your reader won't understand you. By keeping that in mind, you can avoid condensing too much and making your sentences too simple.
Description and mood should usually be interlocked in your writing. Charles Dickens was an early master of that technique. If your hero is in a dangerous place, uses words that suggest sinister surroundings. For example, if he's at a circus and knows there is an unknown murderer somewhere in the crowd intending to kill him, don't describe the circus as a happy place--make the clowns sinister, the midway shadowy, the circus people scowling.
The best thing to remember is that you need enough description to make the reader feel he is there. Some writers use considerable description, some less, and today's readers usually don't want too much. Whichever way works with your style, you must remember that you can't describe everything. For example, if you walk into any room in your house and set down to describe it in detail, it would require several pages--there's a ton of stuff in the world. You must pick and choose.
As an example, imagine walking into someone's office. There is a bookcase behind the desk with three hundred volumes within. Obviously, you're not going to describe every book, but you could describe one or two that help reveal the owner's character. In fact, that's the easiest way to do a description--if he smokes mention the ash tray; if he is a gun collector mention the Colonial rifle in its case above the books.
After you've done the description, try to add one phrase or sentence that brings the whole place to life for the reader, either by showing characterization, revealing an oddity, or encapsulating the entire paragraph. Perhaps a horse's fetlock and hoof, hair intact, serves as a paperweight, or a bowl of melted chocolate sits in a chair. Or diplomas hanging on the wall, shining from being freshly polished. Peter Beagle's I See By My Outfit is a book worth reading as an example of descriptive jewels in every paragraph.
I'm afraid I don't have a book to recommend as far as description, but I hope the above is helpful.