I think one does not have to completely understand a book being read on the first try and that's probably impossible anyway.

Go through a book, see which chapters, themes caught your attention the most, go through them again if they are interesting or you'd like to dive deeper, and so on. If you really like the book but feel like the whole picture is missing or you didn't appreciate certain parts enough, read it again after some time - there's even a saying that really greatly written books get better with each reading.

In this regard, I'd say that rereading is even more effective than summarizing but to each his/her own I guess