The Mind Illuminated by John Yates is the one I see recommended the most, and it's helped me tremendously. It goes through ten stages, from novice to very advanced. I'm still close to the beginning, but the instructions in the first several chapters have helped me make far more progress than I ever had before.

The Science of Enlightenment by Shinzen Young is also quite good, with more of an emphasis on insight rather than concentration meditation. There's also the classic Mindfulness in Plain English.

I do use one app. It lets me set a countdown timer, and plays a quiet gong when time is up. That's it.

https://www.reddit.com/r/TheMindIlluminated/

Because they are paid to be uncompromising about your goals, so that when you’re weak they’re strong and push you to be better.