A book on how to make weapons or improve trade, so that people will try it out.
But after the first 3 items or so(to show that the book is legit) we shift to stuff that accelerates science very much.
Get them to create electricity and radio waves earlier.
Society gets to progress faster.
Tho, who should it be sent to? Someone religious? In that case it'd be better to mask it as a religious text, where god is revealing the workings of the universe and encouraging humans to find out more about his design by observation and experimentation?