It is an odd "making of" book, since it's mostly about the pilot episode (and, to a lesser extent, the first two seasons) -- when it's generally accepted that the series didn't take off until the third year. But I think it's unusually detailed for a book of it's type (it's probably the only book to reference the work of Joan Pearce Research Associates, which inherited the work of de Forest Research on the franchise during NextGen and did every additional series and movie until 2005).
Of course, The Companion is better.
Having seen The Exorcist II, I'm not sure I could invest the time in a book about it's making. Just a colossal mess.