You're somewhat off topic for this Quick Answers forum and any answers are going to be very subjective - what is "best" for me may not be best for you.
If you want to learn Bootstrap then can I suggest that you use bootstrap resources rather than straight CSS. E.g.
bootstrap course - Google Search[
^]. Pluralsight appears on that list, I've used them before and found them to provide good quality courses, but they are not cheap.
Before paying for anything try to find reviews of the book or course - even better, see if there is some sort of rating system - but my comment above still stands, your "best" might not be the same as whoever writes the review