Understanding books and documents
There are many special forms of document created for particular reasons and now in the care of archive collections. While some are rarely found, many were produced in vast numbers over many years and will be found in vast numbers in most archives. In general, archives contain documents that had a lasting value, often proving the current owner's legal rights to a property or a privilege, and it is these legal documents that it is most useful to be able to recognise. There are standard books that can help, and more and more web sites that can help either you to understand archival catalogues, or to recognise the type of document in front of you.
The historical context
Knowing about the place where the document was created will often help in understanding it. Knowledge of customs, dialect, place names and people local to the place where the document was created can make it more comprehensible.
For an introduction to web sites of interest to local historians see the following site maintained by Leicester University Department of English Local History: http://www.le.ac.uk/elh/
For an electronic guide to English place names see: http://kepn.nottingham.ac.uk/