The famous NEC or NFPA 70. NFPA stands for National Fire Protection Association. Every electrician knows this book, or to be more precise, the latest updated book each time, regarding the state he wishes to practice.
Some states adopt partly NEC, some an older instead of a newer version. This makes sense since different states have different needs and priorities. Some might be able to harness alternative sources of power, whereas some others have to rely on simple traditional electricity. After some time, states revise the edition and on which terms they choose to comply and some might reject it (although it is rather unlikely). At the moment all states have at least partly adopted it, but this might change in the future.
NEC is a part of the National Fire Codes series and although it is a book and has no law strings attached to it, it is widely embraced and some regulations consider it mandatory. From wikipedia: “It codifies the requirements in order to safely perform electrical installations into a single standardized source”. Regular inspections take place in order to assure compliance to these minimum standards by all electricians.
NEC made its first appearance in 1897 and since then, it is revised, updated and republished, every 3 years (the next update will be in 2014). This book consists of mainly 1000 pages, consists of 9 chapters and since 1993 it can also be purchased as an ebook through the internet.
Its price is usually around 80.00 $. However, some sources might make it available for online reading, but not for printing or downloading. One source is archive.org. Some government and local websites might also have it available for reading, without registration. It usually depends on the state and its union.
All apprentices spend a great deal of time learning and training on the National Electrical Code. So, after their apprenticeship they should be able to handle safe installations and the theoretical background behind it.