If you know which site you intend to use, put it on here, and one of us can look it up and post when the discount applies. If it does, you might want to check if it would be much cheaper with the book/card.
The savings over 8 days will probably amount to what it will cost you to buy the books and ACSI card, however if you do use it again within the same year, you will be quids in.
Don't forget to leave a review of the French and other European campsites you have visited!
As already said, it depends on your chosen site and when they allow ACSI prices. The book cost me £23 inc p+p from Vicarious Books (a lot on here use them for their ACSI book) so if you're staying for 8 nights then a saving of £3 pn would break even, any more and its worth it. I would say its probably worth it anyway as you could use it later in the season somewhere else, there are currently 14 UK sites that takes ACSI.
To my mind it is not just the cost but the convenience of having the directory.
Although you have an idea where you are going, you may not make it for one reason or another. A quick look in the book and most likely there is another site within half an hour.
Takes all the pressure off arriving at a specific site at a specific time.
DaveS1