If you are interested in Roman Provincial coins, RPC is expensive but a wonderful reference. All the common and scarce types for the early empire are in there, and most of the rare and very rare ones too. However, some were not. Recently a supplement has been published: https://www.academia.edu/11106798/A...3_No_lu_Sikke_1_atıf._ISBN_978-0-89722-333-1_ This link is at academia.edu. If you are not deeply into provincial coins it contents will be too obscure for you, but if you are very deeply into provincials it adds to the corpus the latest discoveries.
A statistic that interests me is how many feet of bookshelf will a set of RPC take when all the volumes have been published. 10? 100? Will any of us see that end? Note this is the third supplement and covers the two published volumes and the partial Vol. VII (Gordian). Provincials are numerous like stars in the sky and new ones pop up like rabbits.