[What's that all about?]
Work product is kept confidential unless it becomes an essential element of a case or controversy.  It also wasn't an issue until Trump fired Comey under highly suspicious circumstances and then compounded it by admitting like "he was a nutjob" and "I was thinking about the Russian thing" not to mention possible attempt to cover up by originally disseminating Comey's firing was because of Rosenstein's memo.
In other words, Comey's memos would never had to come to light but for Trump's actions that could amount to an impeachable offense of obstruction of justice.
IIRC, you could also look to Comey's earlier use of such memos in regards to the Bush warrantless wiretapping event.