I've been involved with the National Geographic Society geography bee for 13 years, first as a parent of bee participants (who made it to the Maryland state bee three times) and later as a bee coordinator.
I have observed that there is no one way or best way to prepare for the bee. Some students really enjoy spending time poring over maps and atlases. Other students, including my kids, are much more interested in the historical and cultural geography. Although there are books about bee prep, most of those are geared towards the national bee, not the school-level bee, and tend to freak kids out.
Not having seen any of the bee materials for the 2017-18 bee, these would be my suggestions:
I would not expect a bee participant would know, or need to know, all of this. But unlike a spelling bee, which often requires a student to learn a lot of words s/he will probably never use or even encounter, the geography bee at least encourages students to learn about things they encounter every day: place names on your mail, in the news, in a novel, on the labels of your clothes, in a history textbook, in a movie, etc.
In general, a child will find a wall map of the U.S. and a wall map of the world more useful than an atlas: an atlas stays closed most of the time, but a wall map is available for a casual glance or for more serious scrutiny whenever it is encountered. A subscription to National Geographic magazine is also a great idea for middle schoolers and older.
Whatever a student does to prepare for the bee should be guided by his or her own interests because drilling is really boring and unlikely to lead to meaningful long-term learning. More importantly, whatever a student learns about the U.S. and the world in preparing for the bee is his or hers to keep forever :-).
National Geographic material:
I also post several maps worthy of discussion each week on my blog and on my Facebook page: