Part of my attempt to provide a bunch of unsolicited, anecdotal evidence that probably doesn't work for everyone.
Of course you already know how to read. But do you know how to read well?
Many people who read a book want to read for entertainment. That's perfectly ok -- it seems like a great way to take a break and enjoy yourself. But many times people pick up a book with the intention to learn something important. If you're one of those people, it's important not to fool yourself, end up not learning anything, and just waste your time. That's how you end up reading for entertainment without realizing it.
This was a big problem for me, and I realized I was wasting a lot of time when I otherwise could have been productively reading books.
While I'm still not the best reader, here's how I think I solved that problem:
I'm choosy about which books I read. There are millions of books in the world. I can't read them all. So I have to be choosy. I think about what I stand to gain from the book. Is it worth my time to read it? Is the book actionable? I personally aim to read books that come to me in reviews, that from a skim of their table of contents look like they'll provide real value to me.
I think about what else I could be doing instead of reading. Reading is great, but in many cases experience can be a better teacher. Moreover, picking up some experience can help me understand and apply the lessons in books better. I try to adopt a "doing-reading" loop, where I read something, act upon it, then read another something, etc., continuing to iteratively improve in whatever skill I'm after. This also helps validate the advice of books.
I'm not afraid to ditch an underperforming book. If I don't like it, it's wasting my time, and it's time to move on.
I consider sources other than books. Books are often the best source of information on any topic, and are often higher quality because they're intensely reviewed. But many blog posts and online resources can be great too. I Ignore them at my own peril.
I read a summary, read the book, then re-read the summary. My favorite loop for retaining the main ideas of the book I read is to first find a high quality summary of the book and familiarize myself with the basic points. Then I read the actual book. Then, when I'm finished, I look back on the summary and remind myself of the key points and think of the examples that came up in the book. Note that I can't just read the summary because summaries often only work to remind myself of the book and not to replace the book's content. (Also note that this summary-read-summary loop might not work well with some books, like textbooks.)
I skim and read actively. If I'm hunting for information, there's no need to read every word on every page. I feel free to skip through parts I already know, or when the author is belaboring the point too much. Conversely, I re-read important sections.
I use Audible, Pocket, and Kindle. Even with the best of intentions, I never actually make the time to read. Instead, I've found much more success with fitting reading into the gaps of my day. I use Audible to listen to audiobooks when I'm exercising, cleaning, or commuting. I use Pocket on my phone to read blog posts when I'm standing in line or in the bathroom. I use Kindle on my computer when I'm between tasks, or waiting for a program to run. Substitute other apps as you see fit, but fill your time.
I take notes during, or at least after. Ideally, I should be taking notes on what I read as I read them. But this usually doesn't end up happening. Either way, I aim to take some time after reading the book to summarize the book and internalize my lessons learned and figure out what it is about my life (and/or my research program) will be different after reading the book. If I can't come up with an answer, I probably wasted my time reading the book.
I slow down to digest the books. Many good books teach a lot at once, and I need to slow down to reflect upon each piece. Many times, this means I have to take some time away from the book to reflect.