Moonwalking with Einstein: The Art and Science of Remembering Everything
They probably did lead to degradation of human memory--not in the underlying capabilities, of course, but in how well most people are able to use them.
There is some interesting discussion of this in Joshua Foer's book "Moonwalking with Einstein" . Before books were widespread learning ways to use memory quickly and efficiently was important to scholars and others who worked with large amounts of information. Afterwards, it was good enough to just go with your raw, untrained memory, and the old memory techniques were almost forgotten.
I highly recommend reading Moonwalking with Einstein. It's about a journalist who got interested in memory competitions and, with a lot of practice, ended up winning the US memory championship. Anyone can do it.