The first thing that should come to mind when you are in need of foreign literature is Anglia bookstore (‘England’ in Russian) which is on the Fontanka river, in front of Word order bookstore. There are sections of foreign fiction and grammar books in many big bookstores (Dom Knigi or Bookvoed) but this store was the first serving the cultural capital with books in English so I think of it as a pioneer, still functioning in the center of the Cyrillic universe. It holds mostly literature in English but not exclusively.
By the way, Russian writer Ivan Turgenev lived in the same exact the building (and wrote ‘Rudin’ here).
My best book finds were connected with the history of St. Petersburg or with the foreign point of view onto my country: for example, I would totally recommend you read Culture Shock Russia: A Survival Guide to Customs and Etiquette or Made in Russia: Unsung Icons of Soviet Design (about 1000 RUR each).
In order to head to Anna Akhmatova garden, enter the main gate of the Sheremetev Palace (Fontanki nab., 34) — but do not enter the palace itself and head straight to the left wing: there is a garden gate to the backyard. Wander around famous St. Pete yards and finally, find yourself at busy Liteiny near Anna Akhmatova’s museum and garden.