Here's two essential Theravada Suttas that you can read again and again and again.
I find my computer screen doesn't make the nicest reading sometimes! I like reading the ebook format on my ereader.
Mahasatipatthana Sutta or "The Four Foundations of Mindfulness"
Often said to be the most important Sutta the Buddha taught. The main message being to be mindful of what is occuring in the body and mind, in order to become aware that all conditioned phenomena are impermanent, unsatisfactory and not-self.
http://www.buddhanet.net/e-learning/mahasati.htm (webpage format)
http://www.buddhanet.net/pdf_file/mahasati.pdf (e-book format)
The Dhammapada or "The Treasury of Truth"
A condensed summary collection of Buddha's teachings in 423 verses covers all the essential principles. This essential Buddhist text is a world religious classic. Each verse is a guideline to right living and practical spirituality.
http://www.buddhanet.net/e-learning/bud ... mapada.htm (webpage format)
http://www.buddhanet.net/pdf_file/scrndhamma.pdf (e-book format)