A question (mostly) for the tenured ones: Does writing a textbook count as an academic publication? I teach history, and also languages for historians. I was offered to publish a textbook of Serbian for historians with a relatively new press (Vernon Press). Is it considered to be an academic publication that may be presented to a tenure committee?
A textbook, if published by a reputable publisher, is certainly an academic publication.
How much a specific textbook is considered by a selection committee depends on the field, the textbook, the country and the selection committee. Thus, we cannot tell you if it's worth writing it just for the purpose of strengthening your application.
It might be worth noting that in the humanities, as far as I know (I'm from STEM), books are frequently a more common form of academic publication with respect to journal papers.