I am finding it frustrating to learn how to authorize my computer to download my books from the cloud. The Mac says its not authorized but doesn't say HOW to do it.

How to authorize a Mac or PC

1. On a Mac, open the Music app, Apple TV app, or Apple Books app. On a PC, open iTunes for Windows.
2. Sign in with your Apple ID.

If you're asked to authorize your computer again, it won't use a new authorization. Apple ID uses the same authorization for the same computer.

https://support.apple.com/en-us/HT201251#authorize

See your authorized computers

To automatically sign in to your Account Information page from your computer, click here. Or follow these steps:

1. On a Mac, open the Music app, Apple TV app, or Apple Books app. On a PC, open iTunes for Windows.
2. Sign in with your Apple ID.
3. In the menu bar on your Mac or PC, choose Account > View My Account
4. Sign in with your Apple ID.
5. On the Account Information page, scroll to the Apple ID Summary section. You'll see the number of computers that you've authorized with your Apple ID. You can’t see the names of those computers. If you haven't authorized 2 or more computers, you won't see this section.

If the number of authorized computers listed is more than what you expect, you might not have deauthorized the computers that you no longer use, gave away, or sold. You need to deauthorize all of your computers and then authorize each one that you still use.

Deauthorize a computer

1. On a Mac, open the Music app, Apple TV app, or Apple Books app. On a PC, open iTunes for Windows.
2. From the menu bar at the top of your computer screen, choose Account > Authorizations > Deauthorize This Computer.
3. Sign in with your Apple ID.
4. Choose Deauthorize.

If you run out of authorizations, deauthorize all

If one of your 5 authorizations is on a computer that you don't use anymore, deauthorize it. If you sold or gave away a computer that's used one of your 5 authorizations, you need to choose Deauthorize All on the Account Information page. Then authorize each computer that you still use.