According to the Oxford Bibliographies entry on the philosophy of memory, "Locke 1971 is perhaps the best introduction to, or overview of, the philosophy of memory; it covers the metaphysics of memory in an accessible manner, as well as various epistemological issues."
It is short, dry, and readable, but not particularly enlightening. I found the last couple of chapters on epistemology the most engaging.
The approach it takes is a bit of a relic -- hard to imagine any contemporary discussion of memory could avoid spending significant time reviewing the neuroscientific understanding.