A generic cover letter doesn't help much. Don't tell me you want a job. Show me why you want *this* job.
What interests you about *this job*? Do you understand what we do? How will you contribute? How I can help you grow?
Please do attach papers if asked:
a) Hunting for your papers is annoying and time consuming.
b) Beautiful work in papers may help you stand out for the short list.
c) It shows me what you most value in your prior work.
On your CV:
A work "in progress" or "in preparation" is often interpreted to mean "something I hope to write some day."
I suggest you make it real with a preprint. Otherwise, maybe leave it off (looks like CV padding).
If you didn't attach copies of your papers, you'll want to make sure they're easy to find. Include the DOI in your CV.
Also ideal if those papers are #openaccess. (We're not always accessing from a university library. Subscriptions can vary.)
If you're applying for an #opensource lab, you might want to consider highlighting your own open source codes, repos, etc.
Either by choosing papers that follow open science practices, or by linking repos in your CV or letter.
NIH rules against links don't count here--make it easier on everyone.