What is the proper Algebraic Notation for a pawn being promoted?
FIDE is not the arbiter of chess-notation.  By far the most common way to express promotion using algebraic notation is e8=Q (note that you do not write P for pawn, only the space it moves to).  This is also the only style supported by PGN, the most widely used format for recording games on a computer.

e8/Q and e8(Q) are also sometimes used, mostly by old-timers or old books.  I have never seen e8Q used (I guess I have seen it, I just never really noticed :) see below)



[Edit] By request, I did a small sampling of the literature I have available.

Of the 22 chess books I own, 14 use algebraic while 8 use descriptive (primarily because they are older books).  Of the 14 algebraic:


8 use the e8=Q style
2 use the e8/Q style
1 uses the e8(Q) style
1 uses the e8Q style


Another 2 use the related symbolic style e8♛



So it looks as those e8Q is not as unpopular as I originally thought, but e8=Q is still overwhelmingly the most popular.