Hi,
How much preparation you need really depends on what your current level of English is and what result you want from the IELTS test. There are a number of good books available - Insight into IELTS, Focus on IELTS and and IELTS High Impact (IELTS HIgh Impact is the most recent book, written after some changes had been made to the exam). The Cambridge series (the 4 volumes you mention) are very good for practice but don't really improve your skills, and avoid using book 1 and 2 as they are now dated - the test has changed since they were written.
Hope this helps - why not become a member?
Patrick