Most of the people who caught the title of this post will likely already know what Appendix is all about. They might think this article is about which of the books are good to read and which ones should be avoided. However, this article is actually about how the concept of Appendix N can alter a person's outlook of what D&D is, perhaps for the worst.
Quickly, for those who do not know what Appendix N is, Appendix N is a list of books that the creator of D&D, Gary Gygax, noted as having influenced the development of D&D. This is can easily seen with the inclusion of the Jack Vance books and the effect they had on the D&D magic system (called appropriately enough "Vancian Magic"). A quick look at the titles shows a lot of the pulp/swords & sorcery style.You can find a list of Appendix N here.
For myself, I have read only a few of the books on the list (4 of the 28 authors noted). This is because I was young when these books came out and couldn't afford them until later in my life. By the time I could start buying my own books, most were no longer being published or my reading tastes had changed away from the pulp genre. Thus my views on D&D were slanted away from the Appendix N books. While many of the D&D game systems were designed to reflect the Appendix N books, the worlds I was building were being built on tropes from other books. I missed the viewpoints of the Appendix N books and I was fine with that.
However, of late I have been reading some of the books on the list...and they are pretty good. There was a reason why they influenced D&D. If you haven't read any of them, I would recommend them. Thus far, none of the books have disappointed.
However, there is a danger that people will start thinking D&D is meant to play out like the books recommended. There is a danger that if you aren't playing games like the books "you are doing it wrong". This is the dark side of Appendix N. D&D is designed to accommodate all sorts of fantasy visions and tropes. To think D&D can, or more precisely must, only reflect the Appendix N books is wrong itself. The style of the books is only one way to play D&D.
Some gaming groups prefer more story driven adventure, where death of a character is a rare occurrence. Some prefer more heroic adventures that are not grim. Some prefer high fantasy instead of low fantasy. Most of the books in Appendix N are grim, low fantasy. This doesn't mean a group can't play D&D that is not grim or low fantasy. It doesn't mean "they are doing it wrong" if they lay in a different style.
So, go read the Appendix N books, then go play D&D however you and your group likes. In fact, go make your own Appendix N list that reflects the influences of your own D&D games.