Maybe "A first course in Abstract Algebra" by Fraleigh available on Kindle for less than $10. https://www.amazon.com/dp/0201763907

(It's not focused on Monoids. It also covers groups, rings, and fields from a mathematical point of view. It's written for an undergraduate math major with no prior experience in these areas.)

My favourite abstract algebra textbook is Fraleigh's A First Course in Abstract Algebra. Unlike the article here, which just dumps you a bunch of definitions, the book introduces each structure with proper motivation. It's not a CS approach though.

https://www.amazon.com/First-Course-Abstract-Algebra-7th/dp/... (the 1-star reviews apply to the Kindle version, not the contents. Just get the paperback and you'll be fine)

If you want a CS approach I suggest learning basic Haskell then tackling the fantastic Typeclassopedia. The downside is you'll be missing on structures/theorems that are super useful in math but not that useful in programming.

https://wiki.haskell.org/Typeclassopedia

Fraleigh's abstract algebra text [1]. Sipser's Introduction to the Theory of Computation [2] is also excellent, and possibly of special interest to HN.

[1] http://www.amazon.com/First-Course-Abstract-Algebra-Edition/...

[2] http://www.amazon.com/Introduction-Theory-Computation-Michae...