there is no use saying you've never had a book published, the agent knows that already or you'd have either an agent or at least a previous book referenced
if any of the stories that appeared on the internet are in your collection, you've sunk the deal right there, my husband lost a deal with a major publisher for that reason, that he had previously published the same work on the internet
don't put anything on the internet that you want to put in front of a publisher and it's better to leave out any mention of the internet at all, i can't say this too strongly -- my husband lost the deal in the early 2000s so this information is probably not terribly out of date, although there may be exceptions out there
in fact, the entire last paragraph is weak, you should not be saying things like "the stories are unique and fresh and written w. a strong, distinctive voice," that is playing writer and critic too, if you have a quote from a published writer saying your work has a strong, distinctive voice, give that writer as reference, otherwise leave it off
how i'd do it--
Enclosed you'll find my manuscript, "Stories From the Road and Other Tales of Insanity." I'm a 34 year old truck driver who has worked in this industry for ten years, and my book is a personal take on my experiences and the people I meet along the way.
I've changed all names and identifying characteristics of the people who inspired my stories, and I don't use any real last names.
I have included a self-addressed stamped postcard so that you can let me know when the manuscript has safely arrived. I have enclosed a SASE (self addressed stamped envelope, and if you don't include one, many agents and publishers will throw your package in the trash unread) so that you can let me know what you think after reading my manuscript. I have not submitted this manuscript to any other agent or publisher. (you'll have to leave out this line if you've made previous publishing attempts)
If you prefer to reply by email, my email address is... If you prefer to reply by telephone, you can call at...
Thank you for your kind consideration.
i have not published or even tried to publish a book in a number of years, so my information may be somewhat out of date, but the basic idea of the cover letter is to be short and businesslike
if you have previous publications or references from well-known writers, put them up front, but if you have no publications or references, no need to say so, agents are pretty smart, they can figure it out
be suspicious if the agent hits you up for money
good luck, in my day nonfiction was still possible to sell w.out forking over 10 percent (and it's 15 percent now) to some agent but it has become close to impossible now, so you are wise to take the step of seeking out an agent
hope to see you in the stacks real soon
but the phone number/email is important or at least used to be, back in the day, my editors seemed to just feel better about touching base on the phone