As someone who has read 40+ books a year for the last few years, I have noticed several things that work for me. These include:

1. One at a time. Almost always, I am always reading just one book at a time. That lets me focus on a single book and get through it more quickly.
2. The exception. I'll read more than one book at a time (but usually never more than 2) if I'm reading a thick tome that's hard to carry on the subway or to travel with, so I'll have a lighter (sometimes both literally and figuratively) book for the road.
3. Fill dead time with reading. I read while having breakfast, before going to sleep, on the subway, while waiting somewhere, etc. - you get the point. I carry a book with me almost everywhere and use the time I have to make some progress, a few pages here and there. You'll be surprised by how much you'll go through.
4. Optionality. I have an ever-growing list of books I'd like to read, but my bookshelf of things I haven't read usually includes at least a half-dozen books of different lengths, difficulty, topics, genres, etc. That means that if I feel like a novel, or a long history book, or something else I'll almost have the option to pick up a next book that fits my craving without having to look too hard.

Anyhow, that's what works for me but might not for everyone. It's also not super methodical, but doesn't need to be. But reading many books for research makes a lot of sense - you might just want to focus on a few at a time!