I have a problem how to start a business letter and how to end it.
I will appreciate if you can recomende me a book that can help me
writing excellent business letter/e-mails , it is essential for my work as
a purchasing manager.
Try a google seach for "business letters openings endings" >> business letters openings endings - Google Search
The general principle in writing to a business contact is to use the fist para of the letter to indicate what you are proposing to write about, and the ending para has summary of comments and proposal.