I heard this place has good HSK study materials:

There is a very good book store in Xu Jia Hui. It is on Hong Qiao Rd. right beside Orient Shopping Center. I don t know the exact address, but it should be around 2-6, Hong Qiao Rd.
(By Wangjianshuo)

Slim, yeah, I have found a little one in Xujiahui.
come out of exit 14, start walking down Hongqiao Rd and it very shortly comes up on the left, nearly next to the KFC.
Not a massive collection but there's some choice. It's great for Cchinese langauge learning texts too.

OK, I checked this place out. Just past the KFC, exactly where Paul describes it. Great selection of HSK study books, plus a lot of other Chinese study textbooks. Also a selection of English books, two copies of Wild Swans, Lonely Planet Shanghai's, and a good kids' section. Some periodicals. I spent RMB 134 for five books. The LP Shanghai cost RMB 185, so I picked up a Chinese Eyewitness Guide look-alike instead for RMB 28.

Here's the info on their business card:

SBT Bookstore 上海外文书店徐家汇 Add: 50 Hongqiao Road (虹桥路50号) Tel: 021-6487-3787