Some best sellers fit into more than one category. More than two decades ago Steven Covey's book, 7 Habits of Highly Successful People, spoke to needs in the business world. Even more significant, many buyers considered it a book about values and relationships. Until then, most books were the how-to-make-a-sale type.
Previously I've mentioned 90 Minutes in Heaven, and I believe it expresses a need, and it also spawned dozens of me-too books. People wanted (some would say needed) to know what happens when they die. The various heaven-oriented books provide solace for troubled hearts.
And yet, Don Piper and I have heard from countless readers who resonated with the material about chronic pain. Again, that's what some called a perceived need.