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.

A Directory Of Discarded Ideas by John Grant, 1981.
Picked it up for about 20p from a library book sale when I was twelve. It's a collection of all the mad theories from science that have been proven wrong over the ages. I have re-read it many times, the theories are mental in it and the author has a lot of humour.
Available here;
http://www.amazon.com/directory-discarded-ideas-John-Grant/dp/0906798140
The cover looks different to my copy.
http://img95.imageshack.us/img95/5944/dsc00129w.jpg