This is a select listing of secondary resources for Federal Law topics - since the 52 titles of the United States Code (U.S.C.) cover such a wide range of legal topics (Domestic Security to Internal Revenue Code (tax code) to Telecommunications).
This guide is a starting point if it is unclear what an issue might be categorized under (e.g. a bank cannot resell a foreclosed property until an environmental hazard is resolved) or if there is a question how federal law interacts with state law (e.g. how to weigh the best interest of a client when pursuing a 1983 claim).