title include the page's name, for example on an "About us" page:
About us | Discover the history of our online store | Amazon.com
Or should I just use the keyword filed:
Discover the history of our online store | Amazon.com
If you are not making your document title excessively long by including the name of the page, then yes, you would be well-advised to include it.
Including the page name in the document title will contribute to giving a helpful page descriptor both for humans and for non-human user-agents
From a human reader perspective, it might be useful to keep more explicit detail at the start of the title and push the more general detail towards the end of the title (as in your example above).
So you could have something like:
<title>What we Sell, Our Vision and Our Story | About Us • OnlineBookshop.com</title>