If you can, I think it's more professional to obtain the name of the person responsible for dealing with the bookkeeping to address the letter to. IMO it makes your letter more personal.
When I do sales letters I always decide on what area be it grographical or business type and then phone them to obtain the name of the person responsible for the accounts/bookkeeping (usually owner/manager), and then address the letter to this person and by doing this it makes it more personal. I then handwrite the envelope as this will help ensure that it is opened because people are always more likely to open a handwritten envelope, what they do afterwards depends on the strength of your letter and their need for a bookkeeper.
Good luck and I hope that you get some positive responses.
