D&D 5E SCAG is about $18 on Amazon, if you use the special 30% code, but only until Dec 1st, 2015.

[darjr]

Yes, it is discount over discount. The catch is, that only books with less than a $10 discount qualify for the coupon for 25% -and this second one is capped at $10. SCAG has a $13 discount, and thus is not available for the coupon.

No. The maximum that offer can discount is $10. Over and above any other discount. So if it's a $150 book that amazon has discounted down to $100 then using that code means the price for you is $90. You are correct that it is for only one book.
@CapnZapp

 

[darjr]

So here is an example. I put SCAG in a cart and applied the discount.

Items: $26.46
Shipping & handling: $0.00
Book Promotion: -$6.62
Total before tax: $19.84
Estimated tax to be collected: $0.00
Order total: $19.84

Only good until the 14th.