I have a couple Sapiens but I plan to nestle them in with two or three wall mounted ones. I haven't done it yet, but I think the trick is to simply get L shaped metal wall brackets and put three or four side by side for about 5 inches of width. Once you stack the books, the hardware disappears. Make sure you have a very very strong stud that you are mounting these into - and don't allow too much distance between shelves - probably as little as 6". Books weigh a ton!

CB2 has a cheaper version:

http://www.cb2.com/family.aspx?c=302&f=702&viewall=1

It's $189 and I'm not sure if that's still outside your budget. I noticed that CB2 is opening a store in Union Square in SF. Again, not sure if you're in SF or even CA but maybe you could avoid shipping by waiting for the store to open (July, I think) and pick one up.

Interesting Ikea Hack:

http://ikeahacker.blogspot.com/2007/09/kandell-pilaster-shelf-look-like-for.html

a line up of invisible shelves http://www.poaa.nl/product_info.php?products_id=835

boraxics - that's the same hack i was going to suggest. have yet to try it, but i like the use in the bathroom.

http://www.umbra.com/ustore/product/330633/c560/conceal_book_shelf.html

Umbra in Canada makes individual shelves,
mount a few above each other.

Just so its clear, the "Array" shelf from CB2 does have a slightly lower price point than the "Sapien" from DWR, but it's NOT a knockoff. Both are made in Italy, both are credited to Bruno Rainaldi. They simply use different names.

DWR does sell a taller version that CB2 does not, however.

Based on Sol's link, I'm inclined to think that you could use plain metal bookends as "shelves". Drill screw holes in the base of each bookend, mount them to your column or directly on your wall, and use the flat metal upright of the bookend as your shelf. Stack your books higher than the base tang, and it should disappear pretty convincingly.

Stack your books. Done.

I like the Ikea Hack, myself...I plan on doing this for books and another for towels in the master bath...totally like the look!

As for the concealed shelves mentioned, they are also available at "The Container Store." They have some on display there.

i loved these bookshelves (west elm has one too even cheaper, metal for $169, wood for $179) but at least on the metal one, when i saw it at the store, it looked like the shelves droop. as if the slot they fit in on the spine is too big... wonder if it's the same for the wood one - haven't seen that in person yet. also, just like the ikea hack, west elm has a wall mount one now too: http://www.westelm.com/online/store/ProductDisplay?partNumber=WE-PRODw374&cmsrc=rel&storeId=17001&langId=-1&catalogId=17002&viewSetCode=E

I built my own for about $15. Get one long 2x4 the height of the bookshelf you want, and have another 2x4 cut. If you want all your shelves equal distance apart (ie: 1 foot), then get a bunch of 1 foot pieces. But a plank at least 1/2" thick and however deep you want your shelves. Have this cut as wide as you want each shelf. cut as many shelves as you want.

Steps:
1. Secure the long 2x4 in place to the wall
2. Screw one of the foot long pieces to the long 2x4, starting at the bottom.
3. Take one of your shelves and screw it into the top of the 2x4 (so you'll actually be screwing the shelf down into the top of the 2x4).
4. Take the next foot-long piece and screw it into the long 2x4 on top of the new shelf. Make sure you press down on it so that there's pressure on the shelf. THe shelf is kept in place by the pressure of foot-long pieces, but also by the screws.
5. Repeat till done.
6. I usually also add a 1-2 more screws, through the entire shelf unit into the wall. It doesn't require much though.

more form over function imo (and not good form either imoa)

Actually, it's probably the most useful bookshelf I've ever owned. I don't keep only books on it... I spaced the shelves differently so that I could store various things. And, where there are books, there's no more tilting my head to the side to read the titles! The spines are sideways, so the titles are nice and easy to read. I love it. Truly love it. I built a second one in my kitchen with multiple shelves of different widths and lengths for space for everything from spices to large jars.

Overstock has a version of the tower for $99
http://www.overstock.com/Home-Garden/Spine-Book-Tower/2682148/product.html

In Italy Sintesi have a version of this Bookcase.
I talk here: http://www.asettico.net/?q=node/4