Both my books pay central attention to demystifying interfaces in functional languages. As the discussions with Haskell people have shown, this notion and its importance are not clear to many, and there are too many myths about this topic.
In FDaA, I talk about various interface-like abstractions, free monads included, and mention even more. The whole Functional Declarative Design methodology would not be possible without free monad interfaces. Why not type classes? Again, type classes are not interface-like abstractions; they are genericity-like abstractions, although they can be used as interfaces to some extent. They lack many important properties of a true interface-like abstraction, and do not solve specific problems of software design.
In PTLD, I touch on this topic once again in a couple of dedicated sections:
6.2 Two functional interfaces
6.2.1 Properties of a true interfacing mechanism
6.2.1 Type class versus Free monad
6.2.1 Free monad interface
6.2.1 Type class interface and the Dynamic Payload design pattern
I don’t talk about Backpack in my books. It is a side feature that is marginal enough even in Haskell. My books are not intended to shed light on marginal Haskell features because in general, my books are not about Haskell or functional programming. (But I must admit, PTLD is much more about advanced Haskell than FDaA is.)
However, from what I see and know, the Backpack module system is most similar to OCaml’s module system and can be discussed from this perspective. OCaml’s modules are both interface-like abstractions, and genericity-like abstractions depending on the usage perspective. It seems learning OCaml’s modules can help with Backpack.