О языке древней Руси - И.С. Улуханов Nice, concise little book. It could do with a better layout, but then that's true for more or less anything printed in Russia. It doesn't go too far into detail, so it never get boring ;) A lot of focus is on what parts of OCS found its way into Russian (medieval as well as semi-modern), and very little focus is on OCS structure and grammar.