The problem, as so often in England's school system, is the market.
Reputations stand or fall by exam results achieved, so schools invest heavily in textbooks that help them get ahead in the annual performance tables. Teachers find it hard to resist guides that focus on getting pupils through exams, especially when written by examiners who set and mark the papers. Parents, ambitious on their children's behalf, also like these "crammers" and many buy their own copies to aid revision.
The exam boards' motive is obvious. Two of the three English boards are now tied in with major publishers and naturally want to promote their own courses. From their perspective, having examiners to write textbooks based on courses they are responsible for makes perfect business sense. But the educational rationale is distinctly dodgy. As Philip Pullman says, textbooks that encourage teachers to "teach to the test" must inevitably lead to a narrower, shallower educational experience.
Markets only work well when they are properly regulated and it is the duty of government, through its regulator the Qualifications and Curriculum Authority, to ensure that GCSE and A-level courses are as rich and rewarding as possible. Indeed, the only reason for having competing exam boards is to ensure that pupils and teachers have a choice. The alternative is to have a single exam board, free from commercial ties and politically independent. Neither system is perfect. What is clear is that the advantages of the present one are being rapidly eroded by the rush to publish officially endorsed crammers. That link must be broken; otherwise the exam boards must be taken over and merged.