I have mentioned diversity more than once. Because they are planned in advance, there is no excuse to not include a diverse group of authors, authors from around the world, small press titles, and books about marginalized people. Bestselling, very popular books don't need our help to find their readers. The biggest names in publishing have media appearances and large advertising campaigns along with huge audiences. The nature of publishing in the United States is that it is dominated by people from similar backgrounds. Our job is to find titles that need the spotlight shown on them.
Diverse books are part of any library's collection. Authors like Colson Whitehead write books that win awards and draw attention from readers. Most library's have collections which include his books. A walk through your stacks will reveal books by authors of color, LGBTQ authors, immigrant authors, and works in translation. Unless your collection development policy or purchasing rules are extremely strict, you will likely find small press titles as well. When these titles leave the new books area, they often get lost in the stacks. The new book area is a curated, smaller group of titles which does allow more attention to fall on the books shelved there.
The truth is that we should actively work towards expanding the diversity in our book displays and book lists. If you constantly struggle to fill in a display or a list with books that are written by people are underrepresented then your collection development may need to be reviewed. The world is a big place filled with a huge variety of people. Don't presume that your patrons' interests are limited to books about people just like them. Feed their curiosity by displaying books that will introduce them to someone or someplace new.
One way to increase your ability to provide the best readers advisory service to your patrons is by having your staff take a new ALA eLearning course called Actively Anti-Racist Readers’ Advisory Services. It's a two part webinar taught by Becky Spratford of RA for All and Robin Bradford, a collection development and readers advisory expert. The course is designed to change your mindset and "learn how to help your entire organization craft an actionable plan to seamlessly incorporate the values of equity, diversity, inclusion, and belonging into your regular RA practices." Moving from having a passive approach where racism is acknowledged as something to be avoided to becoming someone who is active in promoting, sharing, and being an advocate for the underrepresented is a powerful transformation. I have taken course from Becky and Robin before and I encourage you to register for their course.