I always take self-help books with a generous pinch of salt and have yet to read one that's fundamentally changed my life, but the better ones can be good reminders of things we know but have forgotten along the way.
The gist of this book is to provide some pointers around how to make positive habits stick and get into the mindset of small, incremental improvements (around anything you like - exercise, work, character traits, etc.), as well as how to ditch bad habits.
For the good habits, he looks at ways to make your habit obvious to you (lock yourself down in writing to what you're going to do when and where, and habit stack, e.g. if you want to work on your marriage, 'every time I get into bed at night I'll kiss my husband / wife', or for exercise 'every night I take off my work clothes I'll change into my workout gear', etc.). Conversely, if you want to ditch a bad habit make it invisible - remove your environment cues (obvious one, but get rid of the packet of biscuits if you're wanting to lose weight, for instance).
The next option is to make a habit attractive by pairing it with something you like / enjoy. If I give up my Starbucks Monday to Friday I'll allow myself to buy a new book every Saturday, for instance. Alternatively, join a community that encourages you to stick to your habit through their positivity about the habit (e.g. a really strong gym tribe), as we're mostly naturally primed to want to please people we admire. Inverting that for bad habits, it's reframing your mindset by focusing on all the benefits of avoiding your bad habit (if I avoid this cigarette I am helping myself to avoid an early death, my clothes and breath will smell nicer, I will have money for a holiday at the end of the year with all the money I've saved).
The third rule is make your habit easy so you're more likely to stick with it, especially aiming to downscale your habit to 2 minutes. For instance, rather than the good habit being 'do 30 minutes of yoga' the habit can be ' get your yoga mat out'. The point being if you make it less onerous you're more likely to start and then keep at it. Conversely, make your bad habits more difficult (e.g. lock your phone away in a drawer at 7pm every night if your bad habit is mindlessly wasting hours on social media scrolling).