2. Demonstrate you know the market position for your book. That means being specific about the genre you are writing in, which might include authors who you feel are writing in the same area as yourself.
3. Relevant personal information about yourself. This falls into two areas.
a) External endorsement of your writing - eg anything published even if it's in a different area to your novel, competition short-listings, creative writing courses attended (especially if they are at a high level eg at MA level). This is the more important of the two, and if you don't have any publishing credits try to generate some asap.
b) Personal information where it is directly relevant to the subject of the book eg it's about a stock broker and you're a stock broker. The key word here is 'directly'.
4. A brief description of what you're selling. This may sound obvious, but I've seen several covering letters where it wasn't 100% clear if the book was fiction or non-fiction, or whether it was one book or a series, or even a book at all. "I am looking for representation for my 95,000 word contemporary women's fiction novel, ABC," then a quick description of the plot/contents.
5. Contact details that sound normal. If your email address is firstname.lastname@example.org then you need a new email address. This is a business letter, and you should sound as professional as you can. Even if you are a fluffy bunnikins sort of person, now is not the moment to tell them.
And don't forget to put in the return envelope and postage....