Elijah is about to embark upon an exciting, and potentially dangerous, adventure with Mr. Leroy. They're going to head over the border into America to find Zephariah W. Connerly III, a self-proclaimed preacher who's stolen the money that Mr. Leroy was going to use to buy his family out of captivity in the South.
As one can imagine, such a perilous journey is fraught with all manner of danger. Elijah needs to make sure, then, that he's properly prepared, and that he takes with him some useful items.
In actual fact, however, Elijah doesn't end up taking all that much with him, not least because he's in a bit of a rush, what with his parents just about ready to leave the house. Even so, what he does manage to put in his bag could turn out to be pretty useful on his journey.
First and foremost, he puts twenty of his best chucking rocks into his sack. Elijah is an expert at throwing rocks, so much so that he can hit fish on the head from a distance. Such remarkable accuracy could serve him well on his forthcoming adventure.
Elijah also grabs a knife, Mr. Taylor's knife to be precise, which he pulls out from underneath his mattress. In the moonlight, Elijah practices stabbing at a paddy-roller before putting the knife in a bag. It's almost certain that he doesn't want to have to use it, but it's better to be safe than sorry.
The use-value of the book that Elijah brings with him is probably negligible, but you never know. However, the piece of paper given to him by Mr. Highgate is very useful indeed: it contains the address of a man that he and Mr. Leroy need to meet when they arrive in Michigan.