Umbraco service to load ebooks as content.

bookcontent-bookservice.cs

using HtmlAgilityPack;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Umbraco.Core.Models;
using Umbraco.Core.Services;
using VersOne.Epub;

namespace Our.Umbraco.BookContent
{
    public class BookService
    {
        private readonly IContentService contentService;

        public BookService(IContentService contentService)
        {
            this.contentService = contentService;
        }

        public void AddBook(int rootId, string file)
        {
            EpubBook book = EpubReader.ReadBook(file);

            var bookRoot = CreateBookFolder(rootId, book.Title);
                       

            if (bookRoot > 0)
            {
                /*
                foreach(var item in book.Content.Html)
                {
                    CreateBookFiles(bookRoot, item.Key, item.Value);
                }
                */

                for(int i = 0; i < book.Navigation.Count; i++)
                {
                    EpubNavigationItem next = null;
                    if (i < book.Navigation.Count - 1)
                        next = book.Navigation[i + 1];

                    // CreateChapter(bookRoot, book.Navigation[i].HtmlContentFile, book.Navigation[i].Title, book.Navigation[i].Link, next);
                    CreateChapter(bookRoot, book.Navigation[i], next);
                }

            }
        }

        private int CreateBookFolder(int root, string title)
        {
            var bookFolder = contentService.Create(title, root, "bookFolder");
            if (bookFolder != null)
            {
                var save = contentService.Save(bookFolder);
                if (save.Success)
                {
                    return bookFolder.Id;
                }
            }

            return -1;
        }

        private void CreateChapter(int root, EpubNavigationItem currentNav, EpubNavigationItem nextNav)
        {
            var chapter = contentService.Create(currentNav.Title, root, "bookChapter");
            contentService.Save(chapter);

            if (currentNav.NestedItems.Any())
            {
                for(int i =0; i < currentNav.NestedItems.Count; i++)
                {
                    EpubNavigationItem next = null;
                    if (i < currentNav.NestedItems.Count - 1)
                        next = currentNav.NestedItems[i + 1];
                    CreateChapter(chapter.Id, currentNav.NestedItems[i], next);
                }
            }
            else
            {
                CreateChapter(chapter, currentNav.HtmlContentFile, currentNav.Link, nextNav?.Link);
            }
        }

        private void CreateChapter(IContent chapter, EpubTextContentFile content, EpubNavigationItemLink start, EpubNavigationItemLink end)
        {

            HtmlDocument doc = new HtmlDocument();
            doc.LoadHtml(content.Content);

            var text = new StringBuilder();
            var startNode = doc.DocumentNode.SelectNodes($"//*[@id=\"{start.Anchor}\"]").FirstOrDefault();

            if (startNode != null)
            {
                var current = startNode;
                while(current != null)
                {
                    text.AppendLine(current.OuterHtml);
                    current = current.NextSibling;

                    if (end != null && MatchesAnchor(current, end.Anchor))
                    {
                        break;
                    }
                }

                chapter.SetValue("bookText", text.ToString());
                contentService.Save(chapter);
            }
        }

        private bool MatchesAnchor(HtmlNode node, string id)
        {
            if (node != null && node.HasAttributes && node.Attributes.Contains("id"))
            {
                if (node.Attributes["id"].Value == id) return true;
            }

            return false;
        }
    }
}

Takes the id of the content node you want to add it to, and the path to a .epub file (download some from https://www.gutenberg.org/)

uses VersOne.Epub package (v.3)