i need a large approx 6ft x 6ft free standing (next to a wall) bookcase
id like it to go under the sloping eaves of the room at one end ideally so being able to make/modify to do this would be good too.
what the easiest and most economical way of doing this?
im thinking

a) just buy a nice one, but not cheap.
b)mdf, buy a router and hope for the best(timeconsuming)
c)that conti board stuff for shelfs but make a case for it to go into?
d) real wood but expensive and needs painting etc etc.
e)several cheap and nasty argos/b+q type tall skinny bookcases fitted
together
any other ideas?

Billy bookcase from Ikea, slightly better than Argos / B & Q, fairly configurable and can be modified if you're careful.

mdf (differing thicknesses for different parts, depending on strength), a circular saw for 'shaping', a biscuit jointer (making hidden joins), glue and lots of clamps – plus a big space to make it in…

billy ones are good but they wont be cheap for the size i need.
in the end all that will be seen is the front edge of the shelfs, the rest can be pug ugly really..

I got i-cubes for my vinyl, but they arent that cheap either

Why can't the ends be seen? If it's wall to wall, why not make shelves with intermediate supports as necessary?

plaster bord walls and one end slopes at 45 degree angle, plus allthe weight on it.

+1 on the Billy bookcase. We have several, and they are perfect for the job.