Just Enough Research by Jessica Hall is the most popular design research book. It’s long, but there are good tactically-useful sections throughout.
Traits of a good user research plan
Summarizes what we already know and what we don’t know
Outlines the input (interviews, ethnographic sessions, etc) and the output (personas, design narratives/briefs, journey maps) of the user research activity
Shows what a successful research project will look like as opposed to an unsuccessful research project
Makes it easy for all stakeholders to provide meaningful input on the plan
Is socialized with teams who will benefit from the research
Shows an appropriately diverse group of likely or committed research participants (or exactly how we plan to find participants)
Traits of a bad user research plan
Is not shared with the people who will benefit from the research
Focuses on methods over impact
Is concerned with decisions that have already been made, instead of current and upcoming decisions
Getting stakeholders and downstream teams invested in the plan is as important as the quality of the research itself. I’m a fan of using stakeholder [ x ] signoffs to get people to read and comment on plans, but that’s probably due to our specific team culture and there may be better ways. I’m not a fan of using in-person meetings.
Additionally, great research can be done without creating useful outcomes for the team, so I think it’s important for a user research plan to convince the reader that the research activities will improve the product in tangible ways (reducing specific pains on a team or solving an ambiguous blocking question) instead of merely being an academic exercise.