Multiple opening books in Arena? In Arena, you can select an "Arena main book" which will be the chess opening book that will be displayed in the Book/TB tab.  However, is there any way to merge multiple opening books into that tab?  It would be useful to combine a couple of good normal books with, perhaps, a special gambit book.
PGN files are simple flat text files.  Using a program such as Notepad++ you can simply merge the PGN files containing the openings you wish, into a single PGN file.  It's pretty simple.

That said, if the two books you're merging contain the same openings you will have duplicates.  Accordingly another strategy is to find the largest PGN book of openings you can, and simply add the missing openings you seek.

You can also convert PGN books to other formats such as the native *.abk format favoured by Arena.

Using one book at a time seems to be a natural constraint of Arena. So you will have much more success working within that constraint and adopting the idea of merging multiple books into a single one and using it.