swindon's on lock road in tsim sha tsui has come back to top place for the widest selections, especially after the two page one stores in times square basement relocated to a smaller place on the 9th floor, as shaddock mentioned.
hk book center in the basement of 25 des voeux road was not bad, as mejulian points out.
cosmos books on johnston road (the tram road) in wan chai has a good selection, too. it's midpoint between the wanchai station and admiralty. (this is what JPMeaney and TJlegg were talking about, ie english in upper floor, chinese in the basement).
commercial press in causeway bay used to have a good selection as softbatch pointed out but nowadays i find the selection approaching that of united press (or united press approaching commercial press). the causeway bay branch is at the famous circular pedstrian overpass, where hennessy road becomes yee wo street and victoria park begins.
united press has many branches mostly chinese but the two with decent english book selections are in langkwaifong (actually d'aguilar) beside 7-11 and and on hennessy road beside wan chai station.
there is a small page one above starbucks in langkwaifong (d'aguilar).
there is a new bookstore called xinhua at the beginning of morrisson hill road on the border of happy valley, wanchai & causeway bay. it is multi-storied and quite spacious. unfortunately the selection is not good and the arrangement of the english section (half the floor space) still needs a lot of improvement, but the space is a welcome change from the usual crowded hk bookstores and they operated till 11pm when newly opened in march.
note that all the above english bookstores sell at 15 to 20% above cover price. i agree with kevin as i also stock up on books when in manila or in new york, or buy them off amazon or barnes & noble, send them to US friends's addresses, who then bring them over when they visit. however hk has its own amazon but without the discount, the prices are at cover price at best - it's called www.paddyfield.com. they used to offer free delivery if you live within hk but now have discontinued this. don't bother visiting the store as the display is sparse but if you don't want to pay more than cover price and are willing to wait 2 to 4 weeks after ordering, then paddyfield is your option.
as whatshername says, for even lower-priced books, visit flow used bookstore on lyndhurst terrace beside the escalator every 2 months. as jayinhongkong mentions, coffee and books on park road out in the western district is not as stifling as flow but has a smaller selection than flow so if you are used to flow, coffee and books might be disappointing. the good thing about coffee and books is on weekends it opens till 11pm while flow closes at 7pm.