How do I add custom fonts to the Android app?

The answer depends on the version of FBReader.

If you are using FBReader 3.8 or later, place your *.ttf or *.otf files in a folder on the device's memory card, then select this folder in the “Fonts” section of the Settings dialogue.

In versions prior to 3.8, this setting is located in the “Directories” section of the Preferences dialogue.