How to read without specific purpose?
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.