Ebook: Aspect: An Introduction to the Study of Verbal Aspect and Related Problems (Cambridge Textbooks in Linguistics)

This was the first book I've ever read thoroughly on verbal aspect, and its crystal clear definitions are very easy to follow. It carefully talks about the meaning of aspect, the perfective/imperfective distinction, and pretty much everything you might want to know about this topic (such as related morphology and combined tense/aspect oppositions).

The only reason why I rated it 4 rather than 5 is because many examples have not been properly glossed, and I had to rely on my prior knowledge of many cited languages (which I reckon shouldn't have happened).