If you want to start from a book you can go through my favorite book Spring in Action by Craig Walls.

If you want online tutorials, start from tutorialspoint or udemy.

I personally, watch online video tutorials and read technical documentation, while I am about to start something new. While I was trying to learn spring framework (and spring boot) I watched the following videos:

Link: youtube.com/watch

You can also register the audit courses for free in coursera (if one available, not sure) or else, few other reasonable courses.