This forum and other various blogs available on the net for free are all you need to go through the assessment process. The kind people on here put more than enough information out there to acquaint yourself with what to expect.
The amount of external applicants who get through is proof enough of that and the practice material the OPC provide is actually sufficient.
Why pay for something that can't guarantee you an end result?
I have to agree in part with what Gash is saying insofar as this forum is a good, if not outstanding source of information. However, I have bought many of these books apart from the recent conductor book and have considered them to be quite useful, particularly as a starting point.
I have got my reservations though. I am not sure if the book on the mechanical comprehension test is actually relevant as I am under the impression that only the freight operators are making use of this test now. In common with the practice material sent prior to the tests, the book does not seem to depict a true representation of the actual difficulty of the test.
Again, with regard to the TRP part 2, the book as does the practice material, represents simplified examples of those questions in the actual test. Whereas I would expect to complete a preparation example in 8-9 seconds, I would not be surprised if I was taking almost double the time in the actual test.
With regard to the structured interview, I believe that there is no substitute to having completed the interview beforehand. Second to that, I would value the advice on this forum but you have to be prepared to search high and low in order to gain a total perspective.
I never practiced the Group Bourdon tests in these books because I preferred the download to save me the tedium of marking my own paper.
For my money, I would like to see more examples of the Driver Fault Finding Test and the TRP part 1, but you can practise for the latter by using the book on the rVCE test.
In summary, whilst I may appear to be a little critical, I would not get rid of my books for one minute. They are good for a warm-up and as a useful starting point but there is no substitute for taking part in the recruitment events for real. Using these books in combination with the advice on this forum will help to get you there.
Personally, for next time, I intend to be more self-sufficient.