Research sketching is one of the most important steps in doing any historical illustration work, and that requires good reference materials. In the years B.C. (Before Computers), any illustrator worth his or her salt had a “morgue”, or a collection of photos filed in boxes or file cabinets, usually alphabetically, of any or all things in creation because you never knew what you would be called on to draw.
My four-drawer file cabinet is long gone, but I saved various reference books, including a couple of books of old wood engravings. For the most part pre-dating photography, these books are amazing for the sheer breadth of their content.
I did these sketches, copied from wood engravings in IMAGES OF WORLD ARCHITECTURE, a huge book that is stuffed with beautiful images of stone age ruins, palaces, cathedrals, soddies, middle-class homes, and every conceivable style and type of building from around the world, arranged alphabetically by country, and from ancient history through the end of the 19th Century.
These sketches were done of course from the section on England and were selected for their visual interest and detail. I may at some point incorporate one of these buildings into a finished piece, but the nice thing about reference sketching is that you slowly absorb the very aesthetic of the period, and can then invent with confidence, picking and choosing which elements convey the story and mood most effectively.
I think it’s also important to not simply copy, whether the source material is a photo or artwork, but rather to “learn” the facade, the window styles, or the porticoes, and to understand the parts and how they work together. The goal is not perfect accuracy, but verisimilitude, the appearance that things “look right”.