• A level 1 book is suitable for an absolute beginner - for example, in the case of a book on a programming language, this is a book that's suitable for anyone who's never programmed before, and in the case of a MySQL book it's a book suitable for someone who needs to start off by asking "what's a database and why do I need one?".

• Level 2 books are appropriate for beginners in the subject that's covered, but who have knowledge of other related subjects - taking our example of a programming book again, we judge that a book at level 2 is suitable for someone who's programmed in a different language to the one covered, and is familiar with concepts such as loops and variables, but never the less is a newcomer to the actual subject language of the book.

• If you select a level 3 book, you'll need at least a little prior knowledge of the subject covered in order to make good use of the book. Typically, general reference books on a subject that don't include a significant beginner's tutorial are classes at level 3, and if you're going to be a serious user of a particular technology, you'll want to have access to at least one good book at this level.

• Level 4 books cover more advanced subjects beyond the parts of a subject that are needed by all users, and (unless they cover lower levels too) assume significant prior knowledge and experience. These books tend to be specialist, and you're encouraged to consider teh content carefully before selecting which book9s) are right for you at this level.

• By the time you get to a level 5 book, you're getting very specialist indeed. You're perhaps just one step removed from actually reading the source code of the product concerned to see what it does, and you're unlikely to find anything more technical in a printed form. Level 5 books are highly unlikely to be on the shelves of your local bookstore and will probably be on a longer delivery time even at Amazon.