The BGB is the short for Bürgerliches Gesetzbuch, the German Civil Code. This set of rules governs most parts of private law in Germany. It also contains the basic rules on construction contracts and it is therefore the first book to look into when confronted with a legal questions. As the German Civil Code is not easy to find in English, I have created a page from which you can find the relevant extracts for constuction law: www.germanconstructionlaw.com/civil_code
This and the VOB/B which are standardised terms and conditions for construction contracts in Germany are the key set of rules for any construction project. I will post an English version of the VOB/B soon.