Did Bobby Fischer actually write "Bobby Fischer Teaches Chess"? Did Bobby Fischer actually write his bestseller primer “Bobby Fischer Teaches Chess” (1966, 978-0553130539), or was it written by a ghost writer?

I often ask myself this question because it seems very unlikely (to me, at least) that Bobby could keep interest in writing a book in which half of the book is about one or two-move mates, in other words, very simple tactics, and the other half doesn't go much deeper than that.
Basically, no.

It was co-authored by Stuart Margulies and Don Mosenfelder, and while Fischer may have contributed a little, it is generally accepted that he just lent his name to the project.

Soltis, Andrew (2003). Bobby Fischer Rediscovered. B.T. Batsford Ltd. p. 10. ISBN 0-7134-8846-8.