At the risk of being pedantic and obscure (but when has that ever stopped someone on a blog?) I would use 'memorable' as a description for books that are remembered in an ongoing, iterative way. Such books would have influenced your thinking and, importantly, they would continue to do so, to resonate.
More, your relationship with a memorable book would be a dynamic one. You would regularly compare your lived experience to its contents, and most likely you would do so in dialectical fashion; your understanding of the world and that of the book would inform each other and shape each other.
In a sense, the memorable book would be one that hasn't really been finished. You're still reading it because you haven't, to date, been able to encompass it. You may never do so: it contains living ideas that may continue to grow.
Why would it have these qualities? It would have to contain subject matter of high significance to the reader. But at the same time, this material would be to some large degree intractable, indeterminate and irreducible. The subject's either too hugely expansive or too minutely intensive.
In this way, a memorable book would have some of the quality of your own lived experience, in that you struggle to determine objectively what it means. Its full meaning still hovers outside your grasp, but as a consequence it may provide some form of guidance, you suspect it's leading you somewhere important - or at least, important to you.
In contrast, books that are merely remembered are ones you have read, processed and absorbed. Their contents have effectively become an integral part of your own knowledge. You rarely reflect on them because their ideas now feel like your own and you happen to be satisfied with their usefulness or truth. They now form part of who you unthinkingly are.
That's my definition, overly involved as it may be. And it's why I'd struggle to get to fifteen. On the other hand, by putting the bar this high I may have made it easier to sort sheep from goats. I'll continue to think about which books are for me memorable in this sense and I'll have a go at listing them*.
* This sort of list is, of course, really an excuse for some wonderfully self-indulgent introspection. That's why a Desert Island Discs appearance is so coveted. Not only can you self-indulgently introspect, you do so in front of a rapt audience (or Kirsty Young at least), which validates your meandering down 'My Way' as objectively interesting and worthwhile. The ultimate self-justification. Anyway, if you can't get a watered-down shot of this from your own blog, it's a poor thing.