The Table of Contents is a MUST-HAVE for non-fiction ebooks, like instructional, education, historical, how-to, etc.etc. If you take a better look at this page, you will see the Table of Contents is actually a part of this page, at the top. Click the RETURN TO TOP link here and you'll see it. You can then click BACK in your browser ... or just click on the chapter you want to see.
This is just one way to do it.
Another method is called "frames" or "iframes". That has definite advantages for you, the author/designer, but does have some drawbacks for the reader. The advantage for you is if you need to make changes like adding or deleting chapters, you can do it easily all in one place. Otherwise, you would have to change every page where your links show up. I learned this the hard way when I first started designing websites. A customer would want a tiny change - maybe a different email address or phone number. Well, I might have to change that email address on 87 pages. I switched to frames and had to change it only once ... and it still shows up on every page the reader sees.
The downside is when a reader wants to print a page in which the content is longer than the frame. Only what is visible prints - the rest just vanishes and is not printed. Oh, they can see it on the screen alright - but it just doesn't print.
When designing a website, frames can keep search engines from giving you the results you seek - because when they look at your index page they don't see the content within the frame. However, with ebooks this is not relevant, since your ebook is not a search-engine-compatible concept to start with.
You are probably looking at this page now, within a frame. If not, click on Index. Notice the same Table of Contents is now in a column at the left of your screen. From there you can click on any chapter or section and it will show up in the window at the right side (the bigger one). The left-hand column remains present at all times. If I need to change a link, I need change it only there.