To quote from Lesson 4 of the EAB's "Elementary,..!" textbook:
But if the English texts means "in order to do something", as in Mi iris por saluti lin, meaning "I went (in order) to greet him", or Mi revenis por esti kun vi, meaning "I came back (in order) to be with you", the Esperanto inserts the word por to indicate this intention or purpose (porpose!)
Always check to see whether "in order" can be placed in front of the English "to do something", because if it can you must use por in Esperanto.
So, in your example, if you need more books so that you can read, then you need por, but if "to read" is describing the purpose the book are to be put to, you don't.
However, although the "in order to" test quoted above is a good rule of thumb, but it is not definitive. For example, in mi havas multon por fari the por is needed, but it fails the "in order to" test. Conversely, the must is inaccurate; There are many situations where "to" could be replaced with "in order to", but por can be omitted:
Kiu rabi eliras, ofte nuda revenas - He who goes out in order to rob, will often return naked.
Ŝi tuj kuris bati ŝin. - Straight away she ran over in order to hit her.
...veninta el Kievo, kien li iris preĝi... - ...coming from Kiev, where he had gone in order to pray ...
