If you want a regular bookshelf, there are a couple of them in the ArchiCAD library that may appeal to you. Just double click on 'Object', and search for 'bookshelf'. Find the one you want and alter its parameters, i.e. length and width, then place it in 3D or 2D.
However, if you want to make a custom bookshelf... then you're gonna have to use the morph tool, which I don't recommend...

You see I have been using ArchiCAD for about 2 month and I still haven't got to that yet, go watch some tutorials on Youtube on how to use that tool.