While a cover letter is not required (USCIS will always ask for what it requires) I have seen that attorneys always include one. Why? It makes life easy for the person who processes the application. Here is what I wrote but based on your situation please edit accordingly:
Date
Address
RE: Application for Naturalization (Form N-400)
Dear Sir or Madam:
Enclosed please find my application for naturalization, as I believe I have all the requirements as outlined in USCIS document M-476.
1. Lawful permanent resident for over 3 years, married to and living with a US citizen for over 3 years, and my spouse has also been a US citizen for over 3 years.
2. Lived continuously at my current address since XXXX and have never left the United States for more than 2 weeks.
3. I am also proud of my moral character.
4. I am confident of my abilities in English and Civics.
5. Finally, I am proud to be a United States permanent resident and would consider it a honor to be a US citizen.
The following documents are enclosed with this packet:
1. Signed and completed N-400
2. Photocopy of both sides of permanent resident card
3. Two photographs
4. Check for $
5. Marriage certificate copy
6. Naturalization certificate for spouse
7. Tax returns for A, B and C
8. Checking and savings accounts (held jointly with spouse) statements
I, therefore, respectfully request that your office consider my application.
Thank you for your kind attention,
Sincerely yours,