Perhaps books along the lines of the Oxford series of "very short introductions" to various topics. This includes the masterful Very Short Introduction to Mathematics by Timothy Gowers. But one could imagine narrowly focussed, specific topics in mathematics, perhaps no more than 150 pages, on many of the topics that have been suggested in response to this post. The closest similar suggestion is Mirco Mannucci's "higher mathematics for the impatient."
Alas, unlikely to be commercially viable.