There is a lot to be said for power-saw furniture modification...
Just realised that if I cut down one of the old sets of flatpack GORMM-like shelves in my bedroom I can turn it into a wide 2-shelf unit for my sitting room, to go below the existing book shelves which for some moronic reason start way above the floor, and put the two low bookcases I've just cleared together as a high one where that one was standing. This will save me another trek out to Ikea, which can't be bad. I shall have to measure up and see if it'll work, wish me luck!
Update 5/11/13 - It lives! It lives! I can get a row of Victorian bound volumes on the lower shelf of the cut-down unit, and stack say 2-3 volumes on top, and they'll be much more accessible than they were.
The "stacking two sets of shelves idea" didn't work - the units have different and incompatible tongue and groove arrangements for that - but there's room to set them up separately and clear off yet another set of shelves (this one made of steel held together with bolts) which will be OK for my sister's greenhouse or something.