Building a Shelf Without Spending Much
Where the cheap books actually are, in rough order of how much you get for the money.

A good personal library costs less than people assume and takes longer.
The mistake is buying new at full price and treating a shelf as something to complete. Almost everything worth owning has been printed in enormous numbers and is circulating second-hand at a fraction of cover price. The constraint is patience rather than money.
In rough order of value:

Library sales, then charity shops
Library withdrawal sales are the best value in the country - well-kept stock, sorted by people who know what it is, usually a pound or less. They are irregular and worth signing up to hear about.
Charity shops come next, with the caveat that the chains price by cover and the independents price by instinct, and the independents are cheaper.

Online, carefully
Second-hand marketplaces are excellent for a title you specifically want and terrible for browsing. Postage makes anything under about four pounds a false economy, so buy in threes from the same seller where you can.
For out-of-print and academic titles it is often the only route.
What to spend real money on
Reference books you will use for twenty years, and anything you have borrowed twice. Everything else can wait until it turns up.
The shelf that is worth having is the one assembled slowly. Books bought in a hurry mostly go back out again.
