they make rubbery pads for covering cymbals and the heads of the drums themselves... they dampen the sound significantly. i used to take naps while my roommate practiced. i think a full set of pads can get expensive, though.

also, you can stuff the bass drum with blankets/pillows/etc. without disassembling it, and that's the one that's gonna be knocking the pictures off the walls and the fillings outa yer teeth.

short of soundproofing the apartment, decorating with a lot of fabric and padding and carpeting and thick drapes will go a long way. the bookshelf idea is also a good one.

these are all things your neighbor should have thought of already. if he hasn't, you have a right to suggest them.