You can take the webpages from the WYSIWYG interface of your HTML authoring program and copy and paste them into the WYSIWYG interface of WordPress blog posts or pages. Keep in mind also that you can write directly into the WordPress WYSIWYG interface using it as a word processor. Or you can write in a word processor and copy and paste your chapters into the WYSIWYG interface of WordPress blog posts and pages.
WordPress itself provides you with menus that constitute automatic navigation built into the WordPress website-authoring system. This navigation capability and other capabilities can enhance your webbook easily and efficiently.
In addition, WordPress provides the capability for third-party programmers to create plugins which further extend the capabilities of WordPress itself. Many plugins are free, and others can be purchased, but they all enable you to do things beyond simple HTML5 and CSS.
Perhaps WordPress’ greatest contribution to your webbook is that it provides you with comprehensive navigation. You can do your chapters either as webpages or blog pages, and together with the widget and menu capabilities of WordPress, you can have navigation on every page in a right column or left column. In addition, WordPress enables you to show menu bars. And don’t forget that WordPress Search (easy to implement) is a substitute for an index.
The thing to watch out for with WordPress is how it displays its interface on a phone. Remember that 70% of digital book readers read on their phone rather than a tablet, laptop, or desktop, and accordingly the phone format is very important. Many WordPress themes are responsive. That means that they will respond with a different format when accessed by a phone (i.e., one that makes sense and works well on a phone).
The special format for phones is typically linear. That is, text (content) appears first, and the menu (Table of Contents) appears below. By testing your WordPress webbook on desktops, laptops, tablets, and phones, you will come to get a feeling of how things work out with a particular WordPress theme and adjust the design of your webbook to make sense for all users.
In addition, WordPress enables easy embedding of audio, video, and color images in your text. In other words, it handles diverse media well.
The WordPress capability to use plugins is a specific benefit. The plug-ins are simply additional programming created by third parties that extend the capabilities of WordPress itself. Consequently, you can do almost anything you want to in a webpage via the good work of third-party programmers. With HTML you can make your webbook anything you want it to be. You can do the same with WordPress. It is very flexible and is designed for ease of use. And of course, WordPress pages are HTML webpages.
CMS WordPress is actually a content management system (CMS) as well as a website authoring program. Besides just enabling you to create webpages it also provides a lot of extra programming functions that help you to organize and structure your webbook.
Disclaimer
It is not my purpose to glorify WordPress or lead you to believe that it is the prime solution for your webbook. Indeed, I have simply included it is it as an example of what the possibilities are of using a website-authoring system such as WordPress. There is plenty of other software that compete with WordPress and is certainly capable. Presumably you will carefully choose the one that best accommodates what you want to do with your webbook.