The recipe for creating the perfect cover/query letter is simple: give editors and agents what they want, no more and no less. That's it. Basically, you need to prepare a letter that provides what the editor/agent requires and then "gets out of the way" so your writing (as illustrated by your work) stands on its own. Since 1994, Writer's Relief has helped many clients fine-tune their cover letters with great success.
Your letter must follow industry guidelines. It must be only one page (8 1/2 x 11, 24 lb. or better weight letterhead), and it must contain editor/agent contact information. It may or may not contain a biographical paragraph if your writing credentials warrant such, and you must thank the editor/agent for his/her time. You may also include some interesting personal information that will grab the attention of the editor/agent such as: "I live in a 200-year-old registered historic landmark on the edge of a forest."
For Short Prose and Poetry: A cover letter accompanies short prose and poetry. It includes the title of the work in addition to some biographical information. With the cover letter you will include a master of your work, which has been professionally prepared (again, within industry standards), and your SASE (a regular stamp for response only; the sample pages will be recycled). DO NOT include a description of the work(s) you are enclosing; this isn't necessary, and it will likely mark you as an amateur to depart from industry standards.
Query/cover letter which accompanies a book submission: In addition to the information contained in a standard cover letter, the query letter contains a "blurb" about the book. A short paragraph covering the basic premise of the novel or nonfiction work will draw the agent in and will encourage him/her to move on to the short synopsis or outline. (When we refer to query letters here, we are not referring to query letters used to "pitch" ideas for articles or books. These query/cover letters accompany sample pages of a book submission.)
Here are some other important posts about cover/query letters!
Writer's Relief Can Compose AND Prepare ALL Your Cover/Query Letters:
http://writersreliefblog.com/post/Want-Us-To-Create-Your-CoverQuery-Letters.aspx
Checklist For Creating Cover And Query Letters:
http://WritersReliefBlog.com/post/Checklist-for-Creating-CoverQuery-Letters.aspx
More Cover And Query Letter Basics:
http://WritersReliefBlog.com/post/The-Query-Letter.aspx
The Cover Or Query Letter From A Marketing Standpoint:
http://WritersReliefBlog.com/post/The-Query-Lettere28094From-a-Marketing-Standpoint.aspx
How To Handle Salutations:
http://WritersReliefBlog.com/post/Salutation.aspx
Self-Publishing: When (Not) To Include Your Self-Published Book In Your Query Letter:
http://writersreliefblog.com/post/Self-publishing-When-(Not)-To-Include-Your-Self-Published-Book-In-Your-Cover-Or-Query-Letter-Bio.aspx
How To Write And Send E-Queries:
http://writersreliefblog.com/post/E-queries.aspx