It’s hard to choose “one piece” of knowledge, so I would try to persuade whoever is writing the book to include the time traveler’s cheat sheet. https://i.imgur.com/O6vSrvq.jpg

I didn’t make it, but includes a lot of information that didn’t have an intuitive path to discovery, but a lot of practical benefits for humanity. If I were to add to it I would try to include at least descriptions of a few other things:

• batteries
• engines
• simple computers (although this may be more involved than the earlier parts combined, so perhaps just simple logic gate diagrams)
• genes
• a guttenberg press
• lenscrafting
• a world map
• calculus
• special and general relativity (also complicated certainly, but could be useful later)
• and basically as many physics equations as I can think of