Most pleasurable for me is the beginning. The knowledge that for several hours I can submerge myself in a different World and distract myself with the exciting exploits of others, and come of at the end better for it, is very contenting.
Infact it depends what kind of book we are talking about. If the book follows some sort of adventure, like the books of Dumas, Sabatini, Scott, Stevenson, Rowling, or Tolkien to name some favorites, what I said above is quite valid.
However, books like those of my favorite author, Fyodor Dostoevsky, tend to cause me the most excitement at the very end. It can't really be called, "excitement", either, as these are profound and morose pieces of literature, but the feeling of having understood the psychological or sociological profiles these authors develop comes, inevitably, at the end, and after reading a Dostoevsky one has the sense that his or her emotional intelligence has been augmented.
Whenever I read an book like those of Dostoevsky, examining the darkest thoughts of us humans, I feel wonderful for it and also feel able and wanting to do some light reading-R. Louis Stevenson perhaps- as a sort of "cool-down" or relaxation method, but when I finish a great and more moving adventure like the Count of Monte Cristo or Harry Potter, I often find it is difficult to immediately start something else-I need time to accept that the great journey is done.
Perhaps the "high" obtained from a new book even apexes as I am purchasing or taking out the book. There is such a feeling that I am making an investment into my own intellect, as well as my pleasure, that is nice.