A friend suggested:

You know those books that you can’t stop thinking about, won’t shut up about, and wish everyone around you would read? The ones that, if taken in aggregate, would tell people more about you than your resume?

So, per request, this is a "list of books that you recommend over and over... the handful of books that you ENDLESSLY recommend, or refer to, or what have you," but since I have a cold, this is late and somewhat unlinked and VERY non-comprehensive. And I reviewed many of these books at more length in my Reading tag.

• Making Software: What Really Works, and Why We Believe It, edited by Andy Oram and Greg Wilson. If you are a technologist, skim at least the table of contents and you'll see something that will help you work better and/or win an argument.
• Getting to Yes by Fisher and Ury: gave me a framework for doing negotiation, including in those moments I might not have realized were negotiations.
• Perfecting Sound Forever by Greg Milner. Here's Scott Rosenberg's review. I listen to music all the time; this gave me a new dimension on which to appreciate it.
• In A Different Voice by Carol Gilligan. How do you reason about your moral choices? What are the ways we might reason differently?
• The Dispossessed by Ursula Kroeber Le Guin. What personalities, dilemmas, approaches would you see and struggle with if you really tried nonhierarchical cooperative modes of making civilization together?
• The Left Hand of Darkness, again by Le Guin. Such a great road buddy story.
• Steerswoman series, by Rosemary Kirstein - my review & recommendation post.
• China Mountain Zhang by Maureen McHugh. Labor, vulnerability, travelogue, queer love, goats on Mars, entrepreneurship, people finding ways to make our lives work in the aftermath of epochal change.
• The Great Arizona Orphan Abduction by Linda Gordon. A fascinating, awful tale interweaved with explanations of ways religion, race, class, gender, and geography played into what happened.
• Dear Genius, the letters of Ursula Nordstrom. Short review here.
• How To Win Friends and Influence People by Dale Carnegie. My memoir/review.
• Slow River by Nicola Griffith. Like China Mountain Zhang, about rebuilding one's life, engineering, learning to have healthy relationships, and making a place for oneself in a massively screwed-up world.
• Producing Open Source Software by Karl Fogel. The basics that every open source software maintainer should know.
• Notes On Nursing: What It Is, and What It Is Not by Florence Nightingale. Nightingale focuses on executive energy, attention, and putting the proper processes into place such that patients (employees) have the resources and quiet they need to get better (do their work). Once you get to a certain administrative level, instead of solving problems ad hoc you have to think strategically. "How can I provide for this right thing to be always done?"
• Bury the Chains: Prophets and Rebels in the Fight to Free an Empire's Slaves by Adam Hochschild. A really inspiring tale of the British abolition of the slave trade and slavery. Reminds us that social justice battles are winnable. And reminds us of the historical connection between civil rights and women's rights. Hochschild specifically wrote to remind us that activists really can achieve what seems impossible. We've done it before and we will do it again. There will be setbacks and challenges and half-steps and repetitions over and over.
• Ben Franklin's Autobiography. So subtle and perceptive about how to change oneself and how to persuade others, and about the folly you'll run into along the way.
• The Great Gatsby by F. Scott Fitzgerald. Empirically a book that fits the brief. Tells me more about how a certain subset of people think every time I read it.