What are some useful workflows for keeping track of interesting but not immediatly relevant books/articles?
Here are two approaches that work for me. The first is a way to capture the essence of documents that are "very interesting, and may prove useful in some future, as-yet-imagined research project, but not immediately relevant". The second is a way to quickly capture the details of documents and prioritize how soon you revisit them to read in more detail. 

First, you can use tagging in your reference manager to summarize the documents. I use endnote. In the research notes section of endnote I put tags to characterize the reference. For example, I will tag the method (e.g., experiment), theory (e.g., motivation) and so on where relevant. Where possible I will also link the record to a specific paper, for example by tagging it as RP1 (RP1 = research paper 1). I separate tags using semi-colons. When it is a paper that I think I will need to read in the future, I will put VIP, or to read. I might also write a note (which I put in notes section of the reference record) of why it is important to read the paper. All of the above means that I can easily find papers with certain tags in the future, as well as use this meta knowledge to quickly determine which papers are relevant on certain topics - without having to actually reread them to find out.

Second, you can also put it somewhere in a list. I use a prioritised to-do list including an an Eisenhower grid, in evernote. 



To do today

1.

2.

3.

To Do Asap:

1.



To think about

1.

In my case I have 7 lists in my evernote document. As shown in figure 1 above 2 of these are at the top of the Eisenhower grid: To do today and To do asap. Four of them are within the box: High importance/High urgency, High importance/low urgency; Low importance/High urgency; Low importance/Low urgency. I have one below the box called: To think about. If I find a useful paper I will add it to one of these list so that I can find it at some point in the future. Of course the list I chose to add it to will be a function of its relevance.