Winchester college's year 9 entrance exam.

We were told by a teacher at DS2's prep who really knew about entrance exams because he'd been in the job 40 years that it was virtually identical to the Harrow Scholarship exam. It's certainly harder that CE. It's graded A-D and from talking to other parents at Winchester getting C's and D's and the odd B is the norm, few get more than 1 A many don't get any. What makes it difficult is if your hoping for a string off A's, these apparently are only given for a really extra- ordinary performance.
I've no idea what books are useful although a friend who's also a French teacher said a standard GCSE French text book would be good preparation and a book listing all the common French verbs, you need to know all the tenses past imperfect present future future perfect etc and be able to use them in written work and speech for the oral.

I hear it is the hardest exam ever
Then you have fallen for the hype.

So few kids are even able to consider taking it .....
A friends son, who is not academic, passed it no hassle, as do a fair few of the kids from the local prep school.

Has your DS already passed the pretest? If so I would discuss with the registrar, explaining the syllabus and subjects your DS does. In schools that prepare for common entrance but not Winchester entrance the children for Winchester are often placed in the scholarship set from year 8 (higher level probably most useful for MFL, maths and latin if done). If your DS's school syllabus is not CE then I would think his results will be interpreted in that context as they would be for an entrant from a state school. If you wanted to do extra work with him then a lot of people recommend the Galore textbooks.