I don't think you should worry about not reading a full book. I've darted around through many books over the past six months learning a lot but not finishing any of them and coming back to them when I'm curious again. You're just kind of meandering around different ideas and topics, trying to build up a pile of ideas before you can come back to the source of interest. I've only recently started reading semi-frequently because I recognized that I had an interest and affinity to the ideas.

I think if you're not reading for a test, take your time, leave for a bit to digest/integrate what you read, go to something else and then come back to it later on.