Thanks to jabalong for giving us the address to this one:
The Oxfam Shop (Causeway Bay)
11/F, Shop 33, Windsor House, 311 Gloucester Road, Causeway Bay, Hong Kong
Tel: 2730 1666
Opening hours:
Mon - Fri: 11am - 7pm
Saturday: 12noon - 5pm
Sun & holidays: Closed
I called in there yesterday. It's a tiny shop on one of the computer-mall floors of Windsor House. There is a very small selection of second-hand books, just a couple of short shelves' worth. Not worth a visit unless you are already in the area.