Every history class I've ever taken has extolled the virtues of the primary source -- accounts from people who were actually present at historical events as they were taking place, or had access to people who were present at said event.
For example, the journal of a soldier who was present at the Battle of Antietam is worth more than a newspaper account of the battle, which is worth more than a history book written five years later, which is worth more than a history textbook written 100 years later. You understand my point, of course.
I was lucky enough to find Modern Steam Engines by Joshua Rose, which was originally written in 1887, and counts as a decent primary source for extant steam engineering knowledge of the period. For the period circa the 3rd Century BCE Siege of Syracuse, we have Polybius (c. 203-122 BCE), who had access to the survivors of the event and (thankfully) was the father of painstaking scholarly research and objective recording of history. Similarly, for the Final War of the Roman Republic (32 BCE - 30 BCE), the only historian who lived at the time was a Greek named Strabo.
Of course, knowing the names of the ancient sources can only take me so far. This is because probably 60 percent of what was written before the fall of the Roman Empire has been lost for a variety of reasons. Fire, war, age of manuscript itself; you name it, it's a reason for some source to have entirely disappeared.
Case in point: Ctesibius, the father of pneumatic science, wrote a treatise called On Pneumatics wherein his work on that topic was detailed. The text is completely lost, and remains only as a work referenced by later writers (i.e. Hero of Alexandria and Vitruvius). Because we don't have Ctesibius's own writings, there's no way to substantiate the reported theory that Ctesibius created a cannon fueled by compressed air, which would help tremendously with my steampunk aspirations in the aforementioned Roman war.
My historical-fiction writer pals know what I'm talking about (including those writing in Classical Athens, Ancient Egypt, and the Middle Ages), and I'm rapidly discovering that the best resources I have are contemporary scholars who summarize what fragments there are into useful information. Thankfully, we're living in a time when the engineering, science, medicine, and society of the period before the fourth century AD has been exhaustively researched by people who are leagues smarter than me.
Mr. Corby recently suggested The Archaeology of Weapons as a resource. The book catalogs weaponry used from the earliest archaeological evidence in existence all the way up through the knights. Tre useful.
So what about you, my fine blog readers? What do you find is your best resource?