I'm actually surprised to see some of the answers.
I would send back... Nothing.
Why? Because I prefer not to undo my own existence.
By sending back anything at all, no matter how benign u may think it to be, you could unleash a chain of events that could prevent you or anyone you
had ever known from being born.
I'll even give you an example that uses a couple of the items people mentioned, the compass, atlas, and dictionary. For this hypothetical situation
it really doesn't matter which one as any of them could lead to the discovery of North America ahead of schedule.
Because some enterprising young explorer heard rumors of this item and got his hands on it, North America was discovered a full 600 years early.
Things go even better this time than in our timeline because the new explorers and settlers become friends with the tribes of North America.
Yep things sure are looking good until words begins to spread to the tribes of South America that newcomers have befriended the northern tribes and
are giving them gifts that could be used as weapons against them. Now the timeline has been altered in a way you did not forsee, because instead of
the Mayan civilization going into a decline it causes them to spread to the north in search of resources and these new weapons.
Fast forward ahead 600 years to when Columbus was supposed to have landed here, and the first Mayan raiding parties are landing along the coast of
England. After almost 500 years of friendly relations, trading, and military cooperation with the Japanese, along with the worldwide trade network
they had formed, the Mayan military had been transformed into the most feared fighting force that the world had ever seen. With nearly unlimited funds
and access to the finest weapons from all over the world they blend fighting techniques, weaponry, and tactics, combined with large doses of Cocaine
before every battle to ensure overwhelming force that no enemy can match.
Fast forward another 600 years back to present times (roughly) and the mighty Mayan empire, instead of declining and disapearing into thin air as they
had in our timeline, has now conquered the entire planet, every nation either completely overthrown or subjugated. Thanks to their religious ties to
space and the stars they even made it to moon a whole 50 years ahead of us only they never stopped. Their fanatical astronomers demanded observatories
on the far side of the moon, which were used to locate the first primitive civilizations outside of our solar system.
Yes it's a proud day as the first ships are now leaving earths orbit to subjugate these planets as well, afterall the Mayan gods still demand blood.
So thanks to you not only our own civilization as we know it, but countless others will be destroyed or subjugated to appease the never ending
bloodthirst of the gods, and all because you thought it would be "neat" to send a compass or atlas or dictionary or some other item, back into time
with the assumption that mankind would use it for good.