Just look at Iddy. Clueless. He’s just about the furthest thing from a scholar you could imagine.
So please don’t take everything he says for granted. Check it out. Read other sources. And for goodness sake, don’t quote him on your final thesis!
There are plenty of other sites out there dealing with the strange phrases of the English language, and most are more scholarly than this one!
As for books, Iddy struggles with anything more challenging than a good Dr. Seuss. Green Eggs and Ham particularly.
However, he did find Dictionary of Idioms and their Origins useful, written by Linda and Roger Flavell and published by Kyle Cathie Ltd.