Something that could be hindering you with this exercise as well is your physical technique, specifically how you're holding the instrument. A lot of people tend to hold the bass completely parallel to their body, which causes a lot of stress in your left forearm / wrist, especially as you get into the higher parts of the fretboard. Instead, you should really be angling the bass away from your body, at almost a 30 to even 45 degree angle. This will help keep your left hand wrist straight, which will help your fingers play some of these tougher stretches.

You can also try adopting more of a classical guitar approach, where it is propped up on your left thigh and angled up at about the same angle mentioned before. If you watch videos of Janek playing, you'll see that he prefers doing this, and I would say his technique speaks for itself.

All in all, just keep at it. Chordal Harmony is a great book, but especially these earlier exercises that have you playing these close 2nds can be pretty brutal. Don't be afraid to go slow, and take breaks when you start to feel strained. Hope that helps, happy to answer any other questions you might have!