Why does using a custom MarshalJSON method on pointer receiver work when passing []T with and without address-of operator?

Question

I would love to understands why the following sample code for passing both books and &books yields the result of the custom MarshalJSON() method defined on the pointer receiver *Book. I do understand that the method set of Book does not include the custom MarshalJSON() method.

package main

import (
    "encoding/json"
    "fmt"
)

func main() {
    book := Book{
        Author: Author{
            FirstName: "Astrid",
            LastName:  "Lindgren",
        },
    }

    books := []Book{}
    books = append(books, book)

    a, _ := json.Marshal(book)
    fmt.Println(string(a)) // {"author":{"first_name":"Astrid","last_name":"Lindgren"}}

    b, _ := json.Marshal(&book)
    fmt.Println(string(b)) // {"author":"Astrid Lindgren"}

    c, _ := json.Marshal(books)
    fmt.Println(string(c)) // [{"author":"Astrid Lindgren"}]

    d, _ := json.Marshal(&books)
    fmt.Println(string(d)) // [{"author":"Astrid Lindgren"}]
}

type Book struct {
    Author Author `json:"author"`
}

type Author struct {
    FirstName string `json:"first_name"`
    LastName  string `json:"last_name"`
}

type BookJSON struct {
    Author string `json:"author"`
}

func (r *Book) ToJSONModel() BookJSON {
    return BookJSON{
        Author: r.Author.FirstName + " " + r.Author.LastName,
    }
}

func (r *Book) MarshalJSON() ([]byte, error) {
    return json.Marshal(r.ToJSONModel())
}

Here is a link to go playground: https://goplay.tools/snippet/x3vZCUnHcsL

Is it because the values inside the slice books are addressable whereas the single value book is not?