Here -- as best as I can tell -- are the system fonts for selected Android reading systems:
Kindle for Android: (updated 8/2022)
- Amazon Ember Bold (Sans Serif)
- Bookerly
- Caecilia
- Georgia
- Palatino
- Baskerville
- Helvetica (sans-serif)
- Lucida
- Droid Serif
- Open Dyslexic
Note: Kindle/Android does NOT have an option for the user to select Publisher Defaults.
Google Play Books:
- Original (?? -- does this mean Publisher Default?)
- Sans (unclear?)
- Literata -- this is supposed to be the default font on GPB
- Merriweather
- Goudy
- Vollkorn
Note: GPB does NOT have an option for the user to select Publisher Defaults (unless it is "Original")
Adobe DE for Android
- No fonts available to select
Overdrive for Android 3.6.2 (Overdrive is a library reading app which uses a branded version of Adobe DE reader and its DRM). No information is given about which fonts are the reading system's default for the general terms ("Serif, "Sans-Serif", etc).
Book Default
Sans Serif
Serif
Monospace
Sans serif Bold
Serif Bold
Monospace Bold
OpenDyslexic
OpenDyslexic Bold