CLs are always so hard to write, so I went to the library to find a couple resources since the examples on the internet are often pretty lame.
Dynamic Cover Letters by Katharine Hansen has been a really good find. My library only had the 1990 edition, but the woman knows what she's talking about. She used to be a hiring director, so about 2/3 of the book is real CLs that people submitted to her (names & places changed to protect the innocent) and she gives each one a grade, from A to D, and tells you why they're good or bad. The first 1/3 of the book that's the how-to portion is actually pretty helpful, too, with some suggestions I hadn't heard before.
HTH!
Re: Good book on cover letters