Difference between publishing textbooks and publishing research for acquiring tenure I've noticed that a few of my professors have written their own textbooks, but they only did so after they had already published many research articles and presumably become tenured at my university. For this reason, I'm wondering (in general), does writing textbooks contribute to acquiring tenure at a university, or are textbooks just considered the "icing on the cake" of the professor's published research?
I have spoken with a few people who have written textbooks. The universal advice they have given is never write a textbook until after you have received tenure. 

In general, you get very little or no credit for writing a textbook, as it doesn't really help in any of the major "checkboxes" for tenure—research, teaching, and service. In addition, the time that you spend working on the textbook can generally be put to better use in writing grant proposals or working on peer-reviewed manuscripts.

I know from personal experience, when I was a graduate student, that there was an assistant professor in my department who spent all of his time working on a textbook. He let his research languish, and ended up with a textbook but no promotion to associate professor.