Encrypted Notepad - Encrypted Notepad is a very simple to use application providing state of the art...
Readwise - Effortlessly rediscover and organize your Kindle highlights
Notebook PEA - Protects your private notes with a password.
Klib - Kindle & iBooks Highlights Manager
Paranoia Text Encryption - Paranoia Text Encryption encrypts text according to a choice of strong algorithms.
Clippings.io - Organize the notes you make on your Kindle