There is an ever increasing interest in Buddhism and every week there is a new book published. The books deal with one or more important aspect of Buddhism.  But it is critical to have the foundational understanding of Buddhism to make lasting progress towards the goal of liberation. Without the foundational understanding, reading a Buddhist book helps only temporarily. 

The structural aspect of Buddha's teaching competes with the study of science or mathematics. Like the students of science and mathematics,  those interested in studying Buddhism, must not skip the foundational material.

Following are the list of books, a Buddhist must study diligently. 

• What the Buddha Taught - What the Buddha Taught: Revised and Expanded Edition with Texts from Suttas and Dhammapada by Venerable Walopal Rahula. For those wishing to save paper and paper money, a kind soul has made this entire book available on-line at http://www.quangduc.com/English/basic/68whatbuddhataught-02.html
• The Four Noble Truths - http://www.buddhanet.net/pdf_file/4nobltru.pdf
• The Noble Eightfold Path - http://www.buddhanet.net/pdf_file/noble8path6.pdf
• The Seven Point Mind Training -Buddhism with an Attitude: The Tibetan Seven-Point Mind Training

The books listed above are for the beginners as well as advanced. One needs to read them multiple times.