I want to find reading material on testing content on a page, specifically formative usability tests.
i.e. test expectations user have for an email about bank balance from their bank or understanding if the right content is being served on a page.
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You may find this a useful summary on the UX of content as such for starters: https://www.nngroup.com/articles/testing-content-websites/
Specific to testing user expectations of e-mail content, the Nielsen Norman Group have compiled a more in-depth study - albeit with focus on marketing - a PDF of which you can purchase for a fee: https://www.nngroup.com/reports/email-newsletter-method/
If you are trying to get to the bottom of your users' priorities when it comes to the informative value (?) of banking information via e-mail, you may want to consider a card sorting exercise. This can help establish a clearer hierarchy of needs, which can drive the structure of the messages.
With content in general, two interlocked items come to mind; language and presentation. Information conveyed via words is by necessity mostly linear, so you'll want to 'string your sentence(s) together) in a way that supports the above mentioned hierarchy of needs, and then clip and simplify through iteration.
Visual presentation allows you to break the linearity of words a little bit because you have that extra dimension of salience - you can make more important items stand out irrespective of their placement (up to a point). That's why I said interlock.
With e-mailed information in particular, anticipate two technical constraints:
Plain text limitations may work against a design intent too dependent on visual hierarchy. The prototype or mockup with which you then test your message structure should be funnelled through common e-mail apps like Outlook that have plain(ish)-text preview modes, with a button or link to 'download images'. Which, as we all know, doesn't just download images; it gives the message its full CSS polish.
Notifiers in a mobile or tablet environment, if enabled, restrict the initial portion of text revealed, so with the sequential or linear language constrain in mind, ensure the gist or hook of your message appears well before the inevitable text truncation point.
You'll want to pitch your content to those limitations. Assume that your recipients won't bother to download any images unless the rough draft version they see at first engages them sufficiently. Most people's e-mail accounts are flooded with more marketing content than they can realistically consume. Granted, banking and finance content has a seriousness advantage over all the other noise but you'll want your message to appear decisively as non-spam, and that within seconds.
This is what more formal usability testing is all about. You bring in users go through several displays; ascertain what they're expecting and searching for.
Bear in mind that there will be significant legal restrictions about what you can and cannot display regarding financial information delivered via email.
To answer your question directly: the best solution is to: