Once you understand the background and history, it's a lot easier to do research on any particular narrow issue. This is where non-lawyers usually go wrong, when trying to understand a legal issue. That's likely to be a contractual issue, and it helps to understand all of the general rules for interpreting contracts that have been laid down by courts over the centuries. With that basis, it will be easier to understand the particular expirations issue. You can certainly do research without knowing all that background, but just be aware that one case never stands in a vacuum.
Secondary sources are things such as legal encyclopedias or treatises on various topics.
Those sets of books try to cover everything in a given area of the law, to the extent that they have space. These texts, while useful to some extent, try to cover so much territory that they end up being rather general on most topics. In spite of their generality, they usually require several hundred volumes to cover everything. These texts, which also tend to run a couple of hundred volumes, are somewhat more precise since they try to cover only one state. Of course, space limitations prevent them from covering everything in detail.