Historians of any period or subject use original documents in their research. These come in many forms, but all were created during or soon after the time the researcher is interested in.
The phrase ‘rare books’ usually refers to printed books created between around 1450 and 1830. What makes these books special is that they were hand-made rather than produced by a machine and were usually published in smaller numbers than today. Although the content of some early printed books is now available online, researchers are also interested in the physical books themselves.
Studying a medieval manuscript written in Latin is not the same as analysing a twentieth-century photograph or researching early printed books. You may need to understand how the document or book was created, when, where, by whom, and for whom.
You may also need to know about
historic handwriting (palaeography)
foreign languages, such as French or Latin
specialist English terminology used in official documents
searching online or card catalogues
Have a look at the following resources
Using Archives: Guide for the inexperienced, created by the Archives Hub
The online guides from Nottingham University on dating documents, understanding historic money, and weights and measures
A Latin course for beginners from the National Archives
A palaeography course for beginners from the National Archives
Studying early printed books, 1450-1800: A practical guide (Wiley, 2019) by Sarah Werner or her website: Early printed books – resources for learning and teaching