What are "public journals" and why did Lee mention them in his August 8th letter?

They are official books of regular entries of works and activities of an organization.

Explanation:

Public journals are official books of entries/records of works and activities of a public organization which are maintained regularly. They keep an account of all activities and initiatives undertaken by a public organization for public awareness or future reference. Robert E. Lee mentions about public journals in his August 8th letter to Jefferson Davis to highlight that he was aware of the criticisms and discontentment by people against his failures. He says that the public journals have publicized his incompetency, therefore, he earnestly request Jefferson to dismiss him of his position as a commander of the army.