Recently Carl Weshke, the owner of Llewellyn books put up a call for readers, asking them to email him about advanced books, i.e. what they wanted Llewellyn to publish that would constitute advanced material. Oddly enough, Donald Michael Kraig had recently written an article in New Worlds Magazine (A Llewellyn magazine), which seems to contradict with Carl is looking for in advanced magic. In Donald's article, he suggests instead that it's not deeper books, but broader books which are more important, and that there's no such thing as advanced material. I disagree with the sentiments of the latter article. I believe that it is possible to write and publish advanced books on magic and find an audience for them, but it strikes me as odd that there is such a fundamental disconnect, or rather contradiction at work within a publisher that wants to publish advanced works, but also discourages the concept. Granted, it could be argued that DMK doesn't speak for Llewellyn, as opposed to Carl Weshke, but he is an editor there, as well as an author. Then too, there's the question of whether Llewellyn ultimately can publish advanced books. Their focus is on trying to reach the broadest demographic, which is impossible to do with an advanced book. An advanced book is for a smaller audience and will generally utilize discourse that is focused toward that audience. The goal isn't to sell books to as many people as possible, but rather to meet a specific market's need, which necessarily limits the number of people who will buy the book.
Finally, The wildhunt as weighed in on this issue, as well as commentators. The sentiment there seems to be that the focus on advanced magic and esoteric technologies is too limited, with not enough focus on advanced pagan spirituality and theology and philosophy. While I think their is a point to be made for the lack of advanced books on those subjects, I also think it's important to find a way to bring some practical applications to those topics. Thus a book on advanced spirituality or theology could still include some form of practical exercises or magical work that integrates the concepts into the person's life. Then too there are publishers such as Asphodel press who do in fact offer a venue for publishing books on advanced spirituality, etc. The difference is asphodel isn't a big publisher, with access to big box book stores, while Llewellyn is...and perhaps that's where some of the discontent from some authors is, because presumably those big box book stores will sell lots of their books...but let me just say that any book of an advanced topic generally will not sell well in big box stores. The audience has to be reached through different means, through personal interaction, but also through recognizing that your audience probably won't go to Barnes and Nobles to buy a book on advanced magic.
As someone who helps publish intermediate and advanced books on magic, and spiritual practices, what I find time and again is that the sales are not driven by big bookstores, but by knowing the audience and recognizing it's not a large target demographic. And that's okay. It works, because the people who do buy the books usually end up being loyal and continuing to support the publisher, while also spreading word of mouth to other people. The market success is based on knowing what the audience wants, and also accepting that what is wanted doesn't need to sell in large numbers to be a success (though I'll never complain if large numbers sell). The bottom line isn't what's important. It's the value the audience places in the material and also the author's voice being fully listened and allowed to show through in the books that are written.