Non-fiction books you read more than once.

If somebody is willing to read a particular non-fiction book twice or more, then it’s at least worth checking out.  And that helps sift the wheat from the chaff in a plethora of books that could be a potentially HUGE waste of time. I must find more diamonds in the rough.  

Also, if you could also explain why, it would help me (and others) understand the appeal that motivates you to read it more than once.

"The Way Of The Ninja - Secret Techniques" by Dr. Masaaki Hatsumi, "The Illusatrated Tolkien Encyclopedia" by David Day, and those Star Wars information books are all easy to read more than once, are at least pore through countless times. Also, books on the history of weapons and warfare tend to be good reads.