Basic Worksheets on Style, Grammar, and Usage to accompany the Gregg Reference Manual, 10 by William A. Sabin [McGraw-Hill/Irwin, 2004] ( Paperback ) 10th edition [Paperback]
If you're feeling insecure about grammar, The Gregg Reference Manual has some of the clearest explanations available. The examples based on business correspondence, which make them easy to apply to writing on the job.