I suspect that most people looking for a bookcase on the internet are looking for something to do a job (that is, store books) rather than a smart piece of furniture. Therefore price is the primary consideration - provided they are convinced it will do the job required without collapsing.
There are cheap bookcases available, made from MDF or chipboard, but these really are a false economy, as they will not provide long service if fully loaded with books. The best solution is to buy solid pine - and the reason is simple.
MDF and chipboard are both made from particles of waste timber that are compressed and glued together. Under load (in compression, to use a technical term) both materials are remarkably strong. The problem is that, to take the example of a shelf loaded with books, the top surface of the shelf is being compressed but the effect on the bottom surface is to pull it apart (in tension). MDF and chipboard are not good at resisting this pulling apart and will soon sag or fail completely.
Pine on the other hand is equally as good as MDF in compression but also contains long fibres that provide good strength in tension. In other words, these long fibres make it much harder to pull pine apart - which is what happens in the bottom surface of a bookshelf. A demonstration of the strength of pine fibres can be seen in that pine roots can be woven together to form strong ropes for lashing poles together.
All woods contain these long fibres but pine is one of the cheapest woods, so a bookcase of pine is usually the cheapest way to acquire a strong, serviceable bookcase.
