Excellent thread from Historian @KeishaBlain on something we don't discuss enough, how to write good tenure letters. A few additional suggestions:
1) After dept. level, your letter is read by committees, Deans, Provost, President. Write clearly for people outside discipline... https://twitter.com/KeishaBlain/status/1287350133181632512

2) In fields like History that don't rely heavily on citation counts and impact factor, communicate how different journals, presses, and book series rate in your field. Harvard and Princeton UP are great, but they aren't the best in every field and subfield...

3) Most tenure cases are decided on quality and quantity of work to date and future research trajectory. Don't forget to discuss the next project! Committees and administrators want to see that scholars will continue to do important work beyond tenure...

4) It is ok to decline tenure letter requests if you don't have time to write or if you are lukewarm on a candidate's work. If your honest appraisal of the candidate's work is "blah," just decline.

Thanks again to @KeishaBlain for this important thread.