How do I read a textbook or monograph with good comprehension?
You are not alone. Even highly educated native speakers find academic books hard. For the reasons for that see Michael Billig's book Learn to Write Badly (Cambridge University Press, 2013). It might amuse and comfort you.

Assuming you have to read a whole book, the strategy I suggest is to go through the thought process that the author should have gone through before putting pen to paper by asking yourself the following questions in the order given:


What single big question is the book trying to answer?
You would hope to find that stated very early on in the book, in an introduction or preface. Sometimes that may not be clear: that is not your fault.
What is the answer?
Sometimes considerate authors tell you the answer just after they have told you the question. Frequently authors are not considerate in that way. A good place to look is any chapter headed "Conclusions".
What in outline are the author's reasons for that answer?
Chapter headings might provide an answer to that, but sometimes you will have to dig a little deeper, skimming the beginnings and ends of the chapters themselves.


Don't be afraid of skimming the book to try to find the answers to these questions. Until you can answer them, simply ploughing through the book will be hard and unrewarding work.
 4. How does the author support each of the reasons given?
To answer that you will probably need to do a bit of ploughing, but don't expect to understand all the detail first time through. Keep constantly in your mind where the book is going, that is Questions 1-3 above.

When you have done all the above you can go into as much detail as you  need to master the author's whole approach.