Assume you read at a constant rate, so reading for twice as long means you read twice as much. And assume you buy ebooks at the rate you read them, so when you read one ebook you buy one ebook. Hence reading for twice as long means you buy twice as much; and reading for half as long means you buy half as much.
Assume also that you take a fixed-duration journey during which you entertain yourself with your e-reader or tablet.
Time spent reading = journey length - time spent doing things other than reading.
Number of ebooks you buy = reading speed constant * time spent reading
So the relation is:
Number of ebooks you buy = reading speed constant * (journey length - time spent doing things other than reading)
Does this clarify things?