I don't think you've read it right then Blueberry.
PIL's messaged OP's DH to let them know that the money to buy eggs was on the way and that they were also sending a book for DD1 and one for DTD's.
OP thought that as they were sending one book for the twins to share, it must cost twice as much as the book for DD1.
When the books and the money arrived, the invoice for the cost of the books was included and OP realised that the DTD's book was actually a cheaper book than the one for DD, cheaper by £1.
If the OP and her DH had been sent the money to buy books as well as eggs, they would have known how much each book cost when they ordered them, and so wouldn't have been surprised when the invoice arrived. Also I doubt the PILs would have specified that the money was for just two books and that the one the twins were sharing was to cost £1 less than the book for DD1.
I realise now you were saying that it would be ungrateful to spend the money on a book they hadn't been asked to buy, but that wasn't clear from the bit I quoted and asked you about.
The money was sent to buy eggs. The books were sent by the PILs at the same time as the money.
Perhaps it would have been ungrateful for the OP and her DH to buy an extra book with the egg money, but I still think it would have been less ungrateful of them to say "thank you very much for the books and the money for the eggs, we hope you don't mind but we used part of the money to buy another book so the twins had one each" than to ring and say "have you forgotten we have three children?"
Then at a more appropriate moment they could talk to the PILs about treating the girls as individuals.