That's not quite what I mean by motivation. To be more specific I should say 'intrinsic motivation'. I'm assuming the book is actually read in either case, motivated or unmotivated.
Let's take the case of Selfish Gene. You can be strongly motivated to read it because it's something educated people are supposed to have read. This is not 'intrinsic motivation' and will be inadequate—and you're likely to just relay some disconnected facts or partially incoherent babble if asked to e.g. summarize it later.
However, if you were previously interested in genetics and concerned by altruistic behavior in animals as a potential weakness in the evolutionary model—then you're going to easily absorb some important principles from the book which relate to this problem: if you consider genes as what's being preserved, as opposed to organisms, then altruistic behavior makes perfect sense—especially given the way it attenuates based on proportion of shared genetic material (or perception thereof).
Being in that state of already having something that needs to be sorted out by the material you're reading is what I mean by 'intrinsic motivation'.