How to read without specific purpose? Posting for a friend, but in first person for simplicity.

I am a research master student in linguistics of a particular language. In my bachelor thesis and some coursework I specialised on a very specific topic. There is enough to explore in this topic to also write a master thesis on it and continue to do a PhD on a slightly broader topic. I will write my master thesis from February onwards and am now preparing for it with an extensive literature review.

My advisor suggests that it is better to not specialise too early in my career, to avoid having trouble getting grants for other work in the future because all my work so far would have been so specific. Thus he wants me to do a much broader literature review, which would allow me to approach my specific questions from a larger perspective.

I see her points and agree. However, I have trouble reading texts of which most is only sideways related to the specific questions I have in mind. So it feels my reading is very inefficient, because I read relatively little that I can use directly and doubt that much of the other things I read will persist in memory. I find the texts interesting but they do not stick (perhaps because I cannot relate them to enough concrete other memories).

What are some techniques I can use to get more out of my general background reading?
It is pretty much a universal truth that if you want something to "stick" in your mind, you need to work with it. Reading isn't enough as it is too passive and only affects the short-term memory. 

To learn something you need to get it to long-term memory and that actually requires rewiring the synapses of your brain. 

The easy way is to take notes on what you read. Paper notes, handwritten, are preferred as they are more effective at "changing the brain" than typing them. But is is also reinforcement to later transcribe the notes and that will help with retention also. 

For me, the best means of taking notes initially is on index cards. This prevents me from just copying long passages, since the cards are small. I can put a title and a date at the top and maybe a sequence number. If I use just one side for primary notes, I can use the backs for other things. I can rearrange my notes by rearranging the cards. I can insert cards. I can select a subset of cards (say as class notes). I can easily discard a card if no longer needed. 

But even better than notes that just describe what you read, are notes that capture your own thoughts. You can make hypotheses about the text, for example. You can relate them to other texts. 

Another aspect of using index cards is that they come in different colors. You can use the different colors for different kinds of ideas. 

You can also carry some subset of your "idea deck" around with you using a simple binder clip. You can carry a few blank cards with you (always) to capture thoughts on the go. 

But for your specific use, you could use the cards specifically to note both the relationships to your current work and how the new ideas differ and might take you in different directions in future. 

But the bottom line, is to make your reading active not passive. See The Art of Changing the Brain by James E Zull for a description of the brain science behind active learning. I've mentioned this book in other posts here also.