Plant Society Book
I have this book. I love it and read it front to back. Super helpful on every topic, troubleshooting sickly plants, interior designing with plants, pots, watering, and species pages. Plus, the design of the book is just really well executed, and I love the uncoated paper, and the foil.
Other ideas, depending on how much she is into plants or gardening: plant journal, humidifier, pots or planters from anthropology or west elm, a moisture meter, or.... a plant :)