BookNotion is an iOS application that is also available for free.
In order to simultaneously link to the same database from multiple devices such as iPhone, iPad, etc., an upgrade to Pro Plan is required.
|Free Plan||Pro Plan|
|Price (in US)||free||$3.99/month or $39.99/year|
|Save Kindle Highlights to Notion||✔️||✔️|
|Post highlights to Twitter||✔️||✔️|
|Save to same database from multiple devices||✔️|