You will have to 'qualify' your potential clients. For instance, if you sell gravel, you will instantly disqualify pillow makers. Ok?
In order to not waste time/resources, you should have an idea of who uses your product. An introductory letter is a perfectly legitimate and logical way to present yourself and your product.
Use pictures, make it attractive.
So. I make concrete and I see by your letter that you sell gravel. Fine. You're over hurdle one.
Now convince me that your gravel is a) going to make my concrete better, b) going to make my business more profitable, c) going to make my life (and work) easier, d) bring me great success.
Skip everything else. For now.
Follow up with a phone call which includes a brief synopsis of your letter and (this is the hard part) ask them directly for a meeting.
Don't hint, don't fiddle. "May I make an appointment with you to go over the wonderful things I can do for you?" Be direct. Be firm. Be convinced that your gravel WILL INDEED benefit this client.
"Lines", tricks, and bulldozer-style personal presentations (where you charge ahead with your script and never actually TALK to the client) are going to fail. Listen carefully. Repeat your strong points, and probe for their questions.
It may be as simple as "can you take payment 30-days from delivery?" "Can you deliver on 2 hours notice?" Or something that you could never anticipate!
There's more, but... well. You've got the idea!
Go get 'em!
Best wishes