By "... an end ..." here is meant an aim, objective or purpose.
Depending on the reading material, one may read with different objectives in mind.
For example, one often reads to gain information, such as when one reads the morning news in order to find out what's going on in the world. One can also read for the purpose of furthering study of a particular subject, as when one reads a text or an essay. Ultimately, one could say that the purpose of both of these examples is to learn.
At the same time, reading may be done for its own pleasure (whether or not one is reading to acquire knowledge or to further study). When one considers the act of reading in this way, it is the pure enjoyment (of reading) that is the objective. Thus the meaning of your quote: "[r]eading is an end in and of itself."
Student or Learner