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The key to the contract is a meeting of the minds. Did the Contractor change the terms? I'm not understanding what the modifying of signature means.
Per wikipedia, Copyright is a legal right created by the law of a country that grants the creator of an original work exclusive rights for its use and distribution. This is usually only for a limited time. The exclusive rights are not absolute but limited by limitations and exceptions to copyright law, including fair use.
Copyright is a form of intellectual property, applicable to certain forms of creative work.
It is essentially a creator of some work's right to ownership and to prevent others from using that work without a license or permission from the original creator.
Had the contractor signed the original agreement before the subcontractor?
Buying books is not a copyright violation nor is loaning them or gifting them. If you began selling them then you would need publisher or author permission.
Without reviewing both contracts, there is a strong argument that there was not a valid contract as the Contractor made counteroffers and did not sign the original.