My Pi is headless and connected to the wifi network (It's a Raspberry Pi Zero W). If I set it up as a Digital Free Library (As per this link https://learn.adafruit.com/digital-free-library/what-youll-need), will I no longer be able to ssh into the pi wirelessly, as the wifi card is occupied? Since it is a Zero it doesn't have an ethernet port, I'm not sure how to set it up so I can ssh into it whilst it's acting as a wifi hotspot?
Yes, you will still be able to make SSH connections after setting up your RPi ZeroW as a Digital Free Library. The SSH server in your RPi is run from a different "port" (port 22) than the library (probably port 80). No problem!
Here's a short explanation of how ports and interfaces relate to each other