Some people swear by reading two+ books at once. I marvel at people who can do that. It might work for you though.
My recommendation is to set up focus mode or whatever it is on your phone. It's best not to have notifications distracting you.
I'd also recommend not reading on your phone but using an ebook reader or the physical book instead. Use your phone for pulling up definitions, Wikipedia entries, articles etc. that will help you to better understand what's going on in the theory, as needed.
Highlight passages which speak to you, which are of particular interest, which are the important parts of the book if you were to make a short summary, and especially which you don't understand or feel like you could understand better.
Revise the highlighted passages each chapter to help consolidate your learnings and to see if further on in the chapter you now understand the highlighted parts make sense to you (and thus can be deleted) or if you need to do some research, post a question to social media or a reading group etc.
If you find that you're highlighting more than a few passages because you don't understand them in a chapter then that's a red flag. Either the theory is too "deep" in conversations with prior texts (this is something that happens with philosophy) and you probably need to read what came before it in order to really grasp what's being discussed in this book or you might need to read the prior work(s) of the author.
I would recommend engaging in some extra processes to help reinforce the learning. This might mean talking to someone (it can even be a pet!) about your learning. Having a friend who is open to hearing about your learnings is really valuable here - giving them updates on each chapter is great for keeping motivation up and for reinforcing learning. You might want to draft a mini essay or summary of the theory. It doesn't have to be perfect or make sense because you aren't going to submit it anywhere but the act of typing out the learnings can really enrich your learning experience and consolidate the learnings.
I'm a savage when it comes to books. I will underline passages and write notes in the margins. I'd recommend it. Or you can put sticky notes in your books if you prefer to. You can even get transparent sticky notes to place over pages and write on.
I'd also recommend reading groups, podcasts, lectures etc. to enrich your learning.