Your example is an interesting one, as the Reagan administration is a broad topic with many books covering just a small slice. If you are solely interested in the foreign policy of Reagan, you could find books on Iran-Contra, SDI, Grenada, the American response to the Soviet-Afghan war, the Reykjavik Summit etc.
Wikipedia is a great resource and I use it a lot, but it is far from a complete solution for topics with any depth. If I read an article on a president, I'll get a brief overview of their administration as well as a few interesting facts and could probably answer a jeopardy question about them, but I would not claim a strong understanding.
I will agree that not every bit of information can be found in a book, but the ability to comprehend book-level arguments and ideas is critical. They are a necessary, but not sufficient component of learning.