lots of ways to build/setup bookcases....I have done it both professionally and personally for 40 years. If your walls can support it, the cheapest good way is metal standards with snap-in supports...(and some of those can look really decent)(bricks & boards, as you say, are unstable in earthquakes)..most expensive is built-to-fit all finely finished...but $4000??? tsk! Wish I was near Idaho, I'd make you a deal! The problem with freestanding units is that they never quite fit well, and waste space...(maybe the cheap way in the back rooms and some 'nicer' ones elsewhere?) Sagging can be fought with small, cut-to-fit boards between shelves, even thinner than a book and stained to be almost invisible. And a little well chosen stain can make the cheapest boards look VERY nice. If you do use fibreboard, you can get stick-on edging that looks quite elegant...