If I were you I wouldn't buy any book.
I would write a small program for each topic in discrete math. For the topic of set theory I would write a program that perform set operations like unions, intersections, difference etc.
A simple example, take a list of customers that have bought product p1, then a list of customers that have bought product p2. Find out which customers have bought both products and which ones have bought only one product, using set theory.
Come up with your own examples, it doesn't matter if they are not real life ones.
Then, in your daily programming, when facing a new problem you will automatically realize whether you can generalize it to one of the topic in discrete (or other) math.
You can start with this page to browse for topics: Discrete math wikibook instead of buying a book.