Leppin's "Blaugast" and "Severin's Journey into the dark" are the best books to accompany a journey to Prague. Both feature seedy, dubious characters wandering around the old town at the beginning of the 20th century. The books are thoroughly depraved, but very good indeed.
Best places to buy this book are Palac Knih on Vaclavske Nam (the English language section is downstairs) and Shakespeare and Sons on Krymska.
The Altstadt is really quaint and has some really good shops. There are great little squares off it with cafes and market stalls selling food. The Old Town centres around one narrow street running parallel to the river Salzach. It contains good branchs of chains like Benetton as well as shops unique to Salzburg like a shop perpetually selling glass Christmas decorations.
Salzburg's cathedral in the Domplatz is also well worth a visit.
The Old Town is situated across the river from the hauptbahnhof and new town. I'm not sure what number the bus is from the Hbf, just ask at the station.