It does seem like we're devolving into "It'd be great for all this tech", but what could the monks realistically bring back? 101 texts on technology is suspicious.
The best chance I can see is to have 4-6 books, and those are expensive, especially if hand written. Plus, they'll all be in Chinese and therefore they'll need someone to translate them (and if they have technical terms, god help you).
My hypothetical books?
1) Agricultural Primer (I imagine it'll include all the best advice for growing crops for farmers, or managing them for estate-holders, which will include both the Seed Drill, terracing, AND the heavy plow - it'll be known of, even if it isn't worthwhile as a result)
2) Analects of Confucius (I expect that the Chinese would be willing to share those ideas to Da Qin)
3) Metallurgical Primer (For the sweet blast furnace)
4) Buddhist Texts (in fact, a Buddhist Monk coming back would be quite useful for translation)
5) Daoist Texts
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I don't actually think this needs to be a sneaky sneaky job either with the monks. I'd actually wait for the Monks to come back, learn as much from them as possible, and then send a delegation with various Roman texts (maybe some military strategy texts, engineering manuals, Greek philosophies) a general/noble, and a theologian. Essentially an embassy of sorts to create closer ties, with an invitation to do the same with the Romans. A rather comical possibility with this long-range embassy (which other than sharing public news can't do much), is that the Chinese take them up on the offer, and rather than the slow overland route, send a ship (with any undesired Romans on board) to connect them by sea.
hehehe, the idea of a Chinese-Roman Embassy, one in the Nanjing/Shanghai area, the other in Constantinople tickles me.
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The question for me is, how much does this cost, and who would Justinian trust to represent him in China, that he could afford to get rid of?