I would wait a few weeks... maybe she'll get it for you as a Christmas/shower present. OR maybe she's thinking she'll wait until after Christmas, because maybe money is tight for gifts. I would send a thank you for the book; and leave it as that. I wouldn't mention the monitor; or if you do, just hint that you cant wait to use it or something.
Just nothing tacky or pushy. Only reason why i'm saying that is because i've had a similar experience. My mom was talking to my cousin before her bridal shower and was saying oh i'm going to get you x & y. Her budget was tight that month and instead bought her just x, and shipped it in the mail. She received a card a week later; hinting at the item that was supposedly promised to her. What was my mom going to say? Awkward. My cousin went bat crazy after she found out my mom wasn't going to get her it. It was like she was entitled to this gift. Ungrateful.