it should be okay to lay on top, the rule i follow that has never let me down is for every 3 to 4 inches in height you need to dowel.
If you plan to place this book cake on top on its own board, then i would use some dowels, just for the ease of serving.
Please, please, put the book cake on a board & dowel. Do you really want to risk something happening? Anytime we put a cake on top of another, we dowel. Be safe.
Congratulations on your 1st post gram!! Welcome to CC.
When I stack a cake on a sheetcake sometimes I dowel and sometimes I don't. If the cakes are 2 different flavors or if they want to remove the top cake to serve seperately then I dowel (actually I use straws). Otherwise I just stack them. It would be just like a cake with 2 tiers.
For the cake you are doing ~ since you are slicing and filling the bottom layer, I would dowel to keep so much weight from being on the filling.
Be sure that you cut your dowels/straws so they are barely above the surface of your bottom layer. If not, your top cake will stick to your bottom cake when removing it and it will remove a lot of icing. (I hope this makes sense ~ I tend to confuse myself 
.)
Welcome Grams - you will be addicted before you know it!
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