Science fiction with mind-blowing philosophy and hard science, preferably without in-jokes.
I might buy a concrete and practical handbook and recommend it to my friends. For example, I did that with the anti-depression handbook "Feeling Good". However, they wouldn't read it.
Greg Egan, Charlie Stross, and Nick Bostrom's Dragonology were all effective doors to interesting discussions, discussions that indicated my friends "got it". Nick Bostrom's more academic-ish stuff just bounced.
Followup to: The Most Frequently Useful Thing
What's the number one thing that goes into a book on rationality, which would make you buy a copy of that book for a friend? We can, of course, talk about all the ways that the rationality of the Distant World At Large needs to be improved. But in this case - I think the more useful data might be the Near question, "With respect to the people I actually know, what do I want to see in that book, so that I can give the book to them to explain it?"
(And again, please think of your own answer-component before reading others' comments.)