I have quite a few books because I had a wide range of interests over the first 10 years or so when I began again after college. Today though, I do most of my research online except for the Krause catalogue for German coins which is my go to reference, which I actually have a physical copy of and can find what I need a little quicker that way. If you are a US collector with a fairly narrow focus, I've heard lots of good things about the mega red books. You could pick the volume with the series you like. I also think most series have a specialty publication devoted to them written by prominent numismatists. When it comes to grading, "Grading Coins by Photographs" by Q. David Bowers is a favorite and affordable. I'm a very visual person when it comes to learning and it helped me hone my grading skills. I would say, with the upcoming holidays, Whitman publishing often has some great deals on books, so if check them out if you would like to expand your library.
@Joshua Lemons . What a surprise. The Bowers book on using photographs is my pick as a Christmas present this year. I am waiting to see how it differs from the Photograde series. James
Which is why most of my "library" is this. If you're really focused on a series, a ton of information is missing from books (or is incomplete or wrong) that is available in specialty periodicals. My ANA membership gives me access to 140 years of the Numismatist. All of the David Lawrence books on the Barber series for example are online. What's surprising is that as far as I know, there is no specialty group for Mercury dimes despite their huge popularity, and no specialty publications.
I have an ANA membership, but I about I haven't used their resources much! The Newman numismatic portal is also a fun site to poke around.
Not nearly as impressive as some here but this is my little stash. A couple years ago I bought a nice oak lawyers book case off Amazon to store important books and magazines ( to me anyway) so I wasn’t looking for something specific all over the house. Was one of the best moves I’ve made. The big binder up top is National Bank notes 6th edition by Don C. Kelly. Really like what I have. I’ve bought the Mega Reds I wanted. Wasn’t going for all of them.