Book.kt

package com.example.bookscompose.http

import com.google.gson.annotations.SerializedName

data class Book(
    @SerializedName("titulo")
    var title: String = "",
    var category: String = "",
    @SerializedName("autor")
    var author: String = "",
    @SerializedName("ano")
    var year: Int = 0,
    @SerializedName("paginas")
    var pages: Int = 0,
    @SerializedName("capa")
    var coverUrl: String = ""
)

BookHttp.kt

package com.example.bookscompose.http

import com.google.gson.Gson
import okhttp3.OkHttpClient
import okhttp3.Request
import java.util.concurrent.TimeUnit

object BookHttp {
    private const val BOOK_JSON_URL = "https://raw.githubusercontent.com/nglauber/dominando_android2/master/livros_novatec.json"

    fun loadBooksGson(): List<Book>? {
        val client = OkHttpClient.Builder()
            .readTimeout(5, TimeUnit.SECONDS)
            .connectTimeout(10, TimeUnit.SECONDS)
            .build()
        val request = Request.Builder()
            .url(BOOK_JSON_URL)
            .build()
        try {
            val response = client.newCall(request).execute()
            val json = response.body?.string()
            val gson = Gson()
            val publisher = gson.fromJson<Publisher>(json, Publisher::class.java)
            return publisher.categories
                .flatMap { category ->
                    category.books.forEach { book ->
                        book.category = category.name
                    }
                    category.books
                }
        } catch (e: Exception) {
            e.printStackTrace()
        }
        return null
    }
}

Category.kt

package com.example.bookscompose.http

import com.google.gson.annotations.SerializedName

data class Category(
    @SerializedName("categoria")
    var name: String = "",
    @SerializedName("livros")
    var books: List<Book> = emptyList()
)

Publisher.kt

package com.example.bookscompose.http

import com.google.gson.annotations.SerializedName

data class Publisher (
    @SerializedName("novatec")
    var categories: List<Category> = emptyList()
)