One of my favourite Economics textbooks is Economics by J.Sloman. It is authoritative, well written and well designed. For an A - Level student it contains everything you need to know. (There are some chapters surplus to requirements, but these are generally marked)
If you study Economics at University, Sloman, will remain a good guide to understanding the main principles.
There are also several other A Level textbooks to choose from
I particularly recommend this Economics through Diagrams textbook. Not much writing, but its good to have all the diagrams in one place.
AS Economics Textbooks
AS textbooks are not as common, it is not absolutely essential to buy an AS textbook, you might just buy an A Level textbook. It is also worth bearing in mind that GCSE textbooks can be very useful for AS Level. (there is little difference between GCSE and AS level economics in terms of difficulty.