I'm not sure you can say that aspect is a missing feature in the German language. Although aspect is not encoded grammatically it exists on the semantic level. So any scientific textbook on German grammar could be used as a reference. Doing a quick Google search, I found these two books:
- "Determinierung unter Defektivität des Determinierersystems" by Andreas Späth
- "Verbalaspekt und Tempussystem im Deutschen und Polnischen" by Jan Czochralski
They both compare German and Slavic language, in which aspect is a pretty dominant feature.