TL;DR - use an AUX cable.
If you are trying to listen to a book using the iPhone spoken content feature with your car's CarPlay feature, but experiencing lag and broken words, the following steps can be used as a workaround.
Connect an AUX cable from your car to your iPhone and activate spoken content with the book. Your car should automatically switch from Bluetooth/iPhone audio to AUX. You can also manually change your car's audio source to AUX. Now, when you play a book with spoken content (using the aux cable) the lag and skipping should no longer occur.
It's possible the skipping is occurring due to power saving built into the car Bluetooth. When it detects paused (silence) between spoken words it temporarily powers down and then back up for the next word - resulting in skipping. Switching to AUX avoids using Bluetooth and allows a direct audio connection from your iPhone to the car.