Make the top two pages of the cake in advance with gumpaste.
Decide on the size of the rectangular cake then rolled out the gumpaste the
size of the base of the cake and then cut it into two pieces so that it looks
like two pages of an open book; these two pieces will go on top of the cake and
most of the decoration will go on top of these sheets.
Dry the two pieces with the top and bottom tips curled a bit, I also used the centre of kitchen tissue roll( cut in half) and inserted it underneath the sheets so that the sheets will look and dry wavy. IMPORTANT make sure you don’t stretch and pull the sheets otherwise the shape will be distorted.
Once it is dry do whatever decorations you want on it just make sure you are careful because it is very fragile.
Bake the cake and carve it on top to get the wavey effect ( make sure that the gumpaste sheets will fit on top snugly and also think and make allowance for buttercream and fondant that is going to go on top of the cake, careful when you are checking ) and cut out two small triangular bits out from both the sides( little triangular hole, the visible edge of the book cover of the cake will go around this). Cover with buttercream and fondant and use the Dresden tool to mark the sides - so that it looks like the pages in a book and paint the sides.
Now stick the already decorated gumpaste sheets on top and add the book marker and visible edge of the cover. Paint and decorate as required.