If you find yourself in the position of saying "Hey, I have some free time; what should I read?" I recommend focusing first on what you want to know or do, secondly on determining what book and articles are likely to contain useful information, and only then determining a plan for what to read. Here is the process I follow.
For each subject in which I am interested, I maintain a bibliography. The most common way entries are added here is by discovering references to books or articles in other things I am reading.
When reading books in physical media, I usually draw an arrow by references or notes that indicate books I should add to these bibliographies. When I am done with a book or article, I go back through and add any such references that still seem relevant to my bibliography. (I do this at the same time I am transcribing notes and quotations, files of which are also an important part of my system.) I usually include a brief note on why I believe this work will be of value and where I got the reference.
Where this becomes useful is when I am pursuing some sort of study. I have learned that I study best with some object in view, so even if my study is for general education, I prepare a syllabus that presents objectives of some sort (code to write, questions to answer, etc.).
Once I have the objectives prepared, I use my living bibliography files to create a reading plan. Of course, once I really delve into a subject, that plan usually expands and I add more references into both my general bibliography and reading plan.
This process provides me an apparatus to pursue my interests in a structured way to make the most of my time. I don't worry much about what to read next except in the context of some such study.
I have found that without this sort of structure to focus my reading, I am in danger of becoming a dilettante. I don't have an exceptional memory, and if I read without a structure, I easily fall into the trap of cultivating superficial knowledge without ever getting deep enough to answer substantive questions or do original work.
I do, however, want to encourage serendipity, so I subscribe to and read periodicals in variety of fields. I have also found The London Review of Books and book reviews in leading newspapers to be useful sources of references and lines of thought that I would probably never otherwise discover.