Currently, the "send" function is not live on Book.io for Polygon and Ethereum books. Here's how to do it with a MetaMask wallet on Opensea.
1. Navigate to Opensea.io
2. Click "Connect Wallet'
3. Then click "MetaMask" 
4. This will open your MetaMask extension. Click "Next".
5. Next click "Connect".
6. You will now see that your wallet is connected. Click on the connected wallet to open the menu options.
7. Click on "Profile".
8. You will now see your assets, click on the 3 dots next to the asset you would like to send. Then click "Transfer".
9. This will open a new screen, confirm the correct asset is selected, then enter the receiver's address and click "Transfer".