When an Audiobook plays files out of order or is missing tracks

Not a question, but an answer that will hopefully help others. I had a few audiobooks on CD that I had ripped to MP3 a few years ago that I tried to open in the Books app, but which were playing out of order. Here's how to fix the problem.

1. Open the MP3s in the Music app. In my case the MP3s all had the same album information so they opened together. Each one was tagged with a different Disc number, but they all had the same track number. I guessed that this common track number was what was throwing off the Books app (and my assumption was right).
2. Select all of the tracks (use Cmd+A on the keyboard; or select the first track, hold the Shift key, then click the last track), right-click (or Option+click) one of the tracks, and then select Get info. You may see a dialog box pop up that asks "Are you sure you want to edit information for multiple items?" Click Edit Items.
3. In the Details tab that opens, in the "track" property, set the "of #" to the total number of tracks in your audiobook (20, in my case).
4. Note that at this point you can set any other attributes of all of the tracks, e.g., the Artist, Album, and Genre, etc. Note that if you click the Artwork tab you can drag & drop an image of the audiobook's artwork into the Album Artwork box, e.g., from Google Images or from an image on your local hard drive. Note also that you can no longer set the Media Kind property to Audiobook like you used to be able to do in iTunes. But don't worry, there's a way to get that set.
5. Click OK to save all of those common properties for the 20 tracks.
6. Select the first track in the audiobook, right-click the track, and select Get Info again.
7. Enter 1 in the track number field.
8. Click the right arrow at the bottom-left corner of the window to move to the next track.
9. Enter the track number and repeat the process for each of the remaining 20 tracks.
10. Click OK to close the window.
11. Open the Books app and click Audiobooks.
12. Resize your application windows with the Music app on the one side and the Books app on the other.
13. Right-click one of the audiobook tracks and select Show in Finder. This will open Finder with the track selected. The other tracks of the audiobook should also be there in the same location.
14. Select all of the audiobook mp3 files.
15. Drag the selected files on top of the Books app. It should show that it is importing the number of tracks that exist in your audiobook.
16. Once the import is complete you should be able to listen to the audiobook.

If you want the audiobook on your iPhone, use the Finder app.

1. Open Finder
2. Connect your phone with a USB cable.
3. Select your phone from the left side of your finder window.
4. Click the Audiobooks tab
5. Check the Sync audiobooks onto iPhone option
6. Click Selected audiobooks
7. Check the box next to your audiobook from the list
8. Click Sync in the lower-right corner

You'll now find your audiobook in the Books app on your iPhone.

I hope this helps!