Hi Friends!  This may be obvious, but the purpose of shellac applied
to book music is to strengthen the slots wherein ride the keyframe
keys.  As has been said, this doesn't improve the appearance of the
books, but will definitely make them last longer.

I have a tape of Arthur Prinzen making books and it shows his wife
using a paint roller to apply the shellac to the bottom-face of the
books.  I simply use a rather 'fat' artist's brush and apply the
shellac to the inside of the music slots one-by-one.  This works
well, unless you are doing a production run of books. :)

I've used a rather 'thick' mixture of shellac flakes, diluted with
what we in the U.S. call denatured alcohol.  These flakes can be
purchased from many hobbyist woodworking suppliers here, and are the
best form to use for any task involving shellac as they can be mixed
up fresh to any desired concentration.  I use the medium-orange shellac
flakes.

Mix them up really thick, burn off the alcohol, and you have 'burnt
shellac'; for sealing tubing connectors into wood.  Great stuff... and
is exactly what the instrument makers used long ago.

Ron Yost, Paso Robles, Calif.