3/4" is about the minimum for decent looking shelves IME. If trying to keep the cost low, then you can get lots of parts out of a sheet of real wood veneer coverd ply or MDF, and then use just a small amount of real hardwood to make lippings or a "face frame" for the shelves. You would probably get a couple of large book cases out of a 8x4" sheet (say £45 for something with a decent finish).
(by comparison, those small 30" tall 14" wide bookcases I linked to earlier, were made from solid hardwood, and each used most of a 9' length at about £3.50 per foot).