One of the things I really need when I'm working on a story is something that I can actually look at, that will give me an idea of how something looks, works, is sized. Several series of books are chock-full of photos and explanations, and they are invaluable.
The Shire Egyptology series is a good example. It is published in the UK and each book covers a subject - food and drink, household pets, medicine, textiles, weapons and warfare, Akhenaten's Egypt... They are not written by the same person. The photographs and explanations are especially useful.
Their website is here.
Ancient Military history is covered by the Men At Arms series published by Osprey (here is a sample of one of their books on New Kingdom Egyptian military).
I have similar sources for other books; one series set in 1830's Paris was helped immensely by a book of old photographs.
Time-Life books put out Echoes of Glory in two volumes. This is one of the most useful books I have ever encountered. It is separated into sections covering edged weapons, firearms, soldiers' life, the home front. Modern photographs of arms and equipment are paired with period depictions. It has been invaluable to me, being, as I am, rather visually oriented.
In Mourningtide, I write of the effect of the death of a son and brother on his family. One of the characters is Ramesses, the younger brother of the man who died. He became the pharaoh Ramesses II, one of the great rulers of his age. I have seen his statues and photographs of his reliefs, but finding anything that has him pictured as a living person in the flesh is difficult. Fortunately, I have succeeded.
Here he is, Ramesses himself, as depicted by Yul Brynner in the movie The Ten Commandments. He is not a prince in this photo - the golden headband with cobra and vulture tends to indicate that he has succeeded his father. I find it a very enlightening photograph, something that I will have to refer to over and over, I think.
One more item for my toolbox...