Looking for intro combinatorics/discrete math book with the following topics

[Werg22]

1. A discussion of binary strings and strings over arbitrary alphabets
2. A discussion of generating functions and bivariate (or multivariate) generating functions, and their applications to enumeration and and probability problems
3. Graphs and Trees (does not need to be that thorough for this part)

Any recommendation?

Thanks in advance.

Edit:

Posted in the wrong section - should be moved to Science Books Discussion

[durt]

For discrete math in general there's Knuth and friend's Concrete Mathematics. For generating functions there's generatingfunctionology (http://www.math.upenn.edu/~wilf/DownldGF.html), which you can download from the author's website. I suppose if you're interested in binary strings and such you'll want to get into formal languages, automata, etc. My class is using Automata, Computability, and Complexity by Elaine Rich. I don't know if you'll find an intro book that covers all those things you listed. Is there something particular you're interested in?

[Werg22]

I'm mainly interesting in problems involving any kind of strings (over arbitrary alphabets, not just binary), problems generally involving generating functions. For example, I'm interested in seeing how a problem of the like "find the average number of blocks of length 2 among all strings of length 10 over an alphabet of 4 letters {a, b, c, d}", this sort of thing. But from what I've seen so far, there aren't any introductory combinatorics book that look at this type of problem.