Obviously, there has to be some minimal familiarity with what gets dumped or else it will all be wasted. Including textbooks on microprocessor design would be a complete waste, as would books about DNA sequencing (PCR and so on) and modern literary theory. So what’s left? Introductory textbooks in biology and physics and mathematics, especially if the science texts are heavy on experiment design. Mathematics might be a problem if notation has changed radically but I don’t think it has. They would be put off by our notions of rigor and proof (a 20th century invention) but calculus has enough practical use it would be welcome. (It also wouldn’t be entirely new. Every culture mathematical enough to grasp fractions seems to have reinvented an intuitive notion of limits and infinitesimals. Being able to do the math without drawing a huge number of really teensy polygons would be a relief.) I don’t know what a 16th century “natural philosopher” would make of abstract algebra or number theory, however.
A single good text on the history of cryptography would kill the entire field of “hidden writing” until the invention of the computer*: All practical pencil-and-paper cryptographic algorithms are fairly easy to break with modern mathematical methods, including obvious ones such as frequency analysis that eluded people for hundreds of years. Even the mighty Vigenère cipher, long believed to be unbreakable, is pretty easy to crack if you have enough ciphertext and an understanding of the fundamentals. “The Code Book” by Simon Singh would do the job wonderfully.
*(By computer I mean something about as complex as an adding machine, which is what they would invent out of desperation in a clockwork recapitulation of the Enigma and Purple mechanical ciphers. It would come to an arms race between people who otherwise would have been making the first practical clocks.)
Biology would be interesting. They still had hugely strong taboos against dissection or anything else we might regard as effective medical research. The first anatomists were graverobbers, or did business with them, and strolled around battlefields looking for interesting injuries. (The mighty Galen, the father of surgery and the hospital, learned what he knew by caring for Roman gladiators.) An anatomy textbook might be regarded as the product of unholy experimentation at best and outright witchcraft at worst. On the other hand, if it fell into the right hands it would be a respite from the illegal process of figuring it out for yourself.
A book about the germ theory as such would be regarded as blasphemous nonsense. A book about hygiene might get a warmer reception, especially if some disease-struck town tried it out of desperation and it helped. If it worked, it would change the face of Europe (Quite literally: Ever see someone who survived smallpox?) and put a strain on the land to support people who didn’t die of cholera, smallpox, plague, scarlet fever, tuberculosis, or any of the other diseases much in fashion.
That leads me to geography. A map with current political boundaries would be a source of blank stares. A map of the world with unknown landmasses would spark serious land-grabs. A map of the world with resources marked (coal, oil, gold, diamonds, etc.) would spark massive wars. It would also change the entire rest of the world.
That’s just to begin with. A good engineering text would keep them busy recreating guns and bridges and Bessemer furnaces and two-stroke engines, with consequences I can’t even dream of. Ditto chemistry. In fact, I would send back a single text that takes them from coal and oil to aniline dyes to Bakelite and gasoline to modern plastics.