We all have our personal family history records that we keep, as both evidence to support our research and as resources to go back to when we have questions about those particular ancestors or others we might be trying to connect to them.
But there are also research guides and handbooks we can keep handy that can be helpful to all of us in our research. These books can be compilations about what resources are at our disposable, especially resources not widely known and not easily accessed online, such as what archives, libraries or courthouses might be in a particular vicinity and what might be in their holdings. We might have old maps delineating old boundaries and place names for areas we are researching. Another good book to consider would be the The BCG Genealogical Standards Manual, by the Board for Certification of Genealogists. This manual outlines the proof standards that professional genealogists hold themselves to in order to make sure they have done the most exhaustive and comprehensive research using the most reliable data available to determine that the information being included in a family tree is most likely correct. This is the book people use when they're studying to become professional genealogists, but in this day and age when so much unreliable and incorrect data is being shared online in the genealogy community, every person researching his or her family tree who wants to make sure it's as accurate as possible with the data available should consider learning the genealogical proof standard. This manual is a good reference to have on your genealogy bookshelf.
Definitely think about what genealogy reference books would be helpful to your own personal research. There are some, like Genealogical Standards, that might be helpful on all family historian's bookshelves, but there could be reference books more specific to your personal research. For me, having books on the place and government breakdown of Germany is helpful, as is having old maps of the New York City area. What references do you go back to over and over again that would be helpful to have in your own genealogy library?