I personally hope that I will eventually find the time to read Practical Foundations for Programming Languages by Robert Harper

https://www.cs.cmu.edu/~rwh/pfpl/index.html

direct link to a preview of the 2nd edition:

https://www.cs.cmu.edu/~rwh/pfpl/2nded.pdf

direct link to an unabridged version of the 1st edition:

http://profs.sci.univr.it/~merro/files/harper.pdf

since TAPL has been mentioned already: Concepts, Techniques, and Models of Computer Programming[0] (CTM) is a great book on language theory.

The whole book is basically taking a core minimal language (Oz) and extending it in various ways to obtain different paradigms (functional programming, declarative concurrency, message passing concurrency, imperative/stateful programming, OOP, shared state concurrency, constraint programming, logical programming etc).

It's very fun and illuminating.

It also includes a part on formalizing semantics, of which I recall nothing.

[0] https://www.info.ucl.ac.be/~pvr/book.html

Software Foundations seems to be really good.

https://softwarefoundations.cis.upenn.edu/

And it's free.