What are some alternatives?
When comparing Knotes and Nimbooks, you can also consider the following products
Readwise - Effortlessly rediscover and organize your Kindle highlights
Klib - Kindle & iBooks Highlights Manager
Unhoard - Rediscover things from your Pocket every week
Clippings.io - Organize the notes you make on your Kindle
Booxia - Save & review highlights from Kindle & Web pages
Hazel - Lighweight update server for Electron apps