Addresses for materials/application

Hi everyone. I apologize if this is a stupid question, but there are soooo many documents on the website to look through and I have not been able to find an answer to this:

Can someone tell me to whom I address cover letters to? I am writing one for my spousal visa application, as is my husband. We are also obtaining letters from his employer and our landlady. I cannot find who to address the letters to on the website. I assume that when I finish my application, it will tell me where to send my application print-out and supplemental materials, but I'm not sure what to do for these letters that are to be included with those materials. Again, sorry if the answer to this is in an obvious place. I have not yet begun the online application because I do not want to start that until I have all supplemental materials from my husband in hand, so I do not know if it is on that part of the website. Thanks for your help.

Generally, I would just use either:

To Whomever it May Concern
or
To the Entry Clearance Officer.

A sample letter for the sponsor's letter of confirmation is here, so you can see how it has been addressed and what has been written in it (note: the consulate address on it is wrong as it hasn't been updated for Sheffield yet):
http://www.londonelegance.com/transpondia/sites/default/files/spousal.pdf

The address for where to send your documents to might not actually be given at the end of the application, because they may not have changed it from NYC to Sheffield yet (a few people have been caught out by this).

However, you can find the address on this page: https://www.gov.uk/government/publications/usa-apply-for-a-uk-visa/apply-for-a-uk-visa-in-the-usa

And it is:
International Operations and Visas
6 Millsands
Vulcan House
Sheffield
S3 8NH
United Kingdom

Just a couple of notes regarding the letters you're getting written (in case you aren't aware):

The Applicant's Cover Letter (optional)
Should be only about the application forms and documents being sent and no personal relationship information.
It should include:
- the type of visa applied for
- the date the visa should be made valid from (if post-dating by up to 3 months)
- a list of documents included with the application
- an explanation of anything unusual about specific documents, or an explanation of a criminal record or bad UK immigration history

The Sponsor's Letter of Confirmation (mandatory)
This is where you give details of how the visa requirements are met.
It should include:
- his ability to sponsor a visa (i.e. UK citizenship)
- his employment status and how he is meeting the financial requirement
- where you will live together in the UK
- a short description of your relationship
- a list of documents he is including

The Letter from his Landlord (mandatory)
This should just confirm the tenancy and that they are giving you permission to live in the property with your sponsor

The Letter from his Employer (mandatory)
This should include confirmation of:
- his employment and gross annual salary
- the length of his employment;
- the period over which he has been or was paid the level of salary relied upon in the application;
- the type of employment (permanent, fixed-term contract or agency).