Chess Engine Opening book I'm trying to evaluate the differences between Opening books. 

For example: Have Fritz 12 Book and Rybka Aquarium (CTG format). For many of the standard openings. I've tried with atleast main lines of 
a. QGD 
b. QGAcc
c. Nimzo Qc2 
d. Caro Capablanca 

So far couldn't find differences between these 2 - gone as deep as 17-20 moves in some lines. How do these 2 contrast? 

Regards
It's very unlikely you'll be able to compare opening books just by playing against them. Main lines will be approximately same in all books, what you really want to see is how "wide" these books are, do they contain only a few main lines, or plenty of variations as well.

The easiest way to do that would be to convert the books back to the readable (PGN?) format and see how many different games are included for a particular opening.

Might be easier to do that using some software than comparing by hand. Even better if the software can read the books directly, this way you don't lose any information while converting books to .PGN format, but you'll have to be able to program for that to happen, because I don't know any ready-to-use software for the books comparison.