First of all it's a fork of Noto-Emoji which I started after they changed the overall emoji style.

Other sources are:

• The Comic Neue font is (mostly) used whenever some 'traditional' characters are needed (like in ⁉️, 🔙 or 📴).
• The Book emojis (📕, etc.) are inspired by Emojipedia
• The Bookmark Tabs emoji (📑) contains a Tweet by SwiftOnSecurity.
• The Wheelchair emoji (♿) is based on the Accessible Icon Project. You can still see a part of the old wheelchair icon. This style (with a little transparency) is also based on this project.