I'm actively using two libraries (I have another one I haven't started populating yet and plans for at least one, maybe two more). The main library has my books. I came back from a trip yesterday and my main computer refused to boot up (I'll drag it to the shop today or tomorrow) so I'm trying to remember which columns are custom. I have a sub-series column (the use should be apparent) and a format column. I use the format column to determine how many books I've scanned (they are all PDFs). I also have a column to determine which books are unread. No stars means I've read it. One star means I'll read it someday. Two stars means it's higher up on my reading list. Three stars are books on my immediate reading list (such as books in a series I'm currently reading). Four stars is for the book I'm currently reading (at home, I read from my TV screen which is patched into my computer). I have another column to show which books I have loaded onto my JBL e-book reader.
My comic book library uses a custom series column (I didn't want or need a series column that was numbered). I had to add a space in front of the column title to get calibre to let me use Series for the title. I also have an unnumbered sub-series column (the comic title has the issue number in it so numbering the subseries was both redundant and extra work). The only other custom column is the one for unread (I'm not planning on reading any of the comics soon so all I'm using is one star for unread).