You've been way more than patient. The seller can blame USPS all he wants -- HE is responsible for making sure the book is delivered timely.
Immediately open an item not recieved case on eBay. In the meantime, I suggest you buy it from Amazon. If the tracking number hasn't updated in 10 days, the book is probably lost or the guy hasn't shipped it yet. If he ships via Media Mail, it will take another week to arrive.
Be sure to neg the seller when this is all over to warn others.
Tell the seller it hasn't arrived and you will be filing a "not delivered" complaint with Ebay.
It's almost a week late, not two weeks. The due date was Jan 10th to the 15th. After the 15th it's late. Before that, it's just early. It's the 22nd today so it's one week. You can't count from the 10th.
Stop contacting the seller. Open an Item Not Received case and let Ebay handle it. If the book was shipped Media Mail, it can take 2 or 3 weeks. I just did that myself.
I'd contact them again and let them know you need it and that you're very angry. I have had many problems like this with ebay too. One time I ordered a SIMPLE small, locket necklace and it was estimated to take a week to come but it took almost 2 months! When i got it, i didn't even need it anymore because i was buying it as a gift.
The tracking info means it's still in the postal system. Contact the post office about this.