First: I think it's easy to lose sight of the fact that the starting point of all magic is always you yourself. It's not the system or the methods you employ, it's you, the fulcrum of anything that will or might happen. As this fulcrum evolves (or degenerates, which is also possible), so will your understanding of magic. To my mind, practitioners are much too outward-looking in this field - this works and that doesn't, this stuff is garbage and that one isn't, etc. People change over time, and some make it their principal concern to transform themselves, so what you believed to be impractical and absurd in the past may work for you after all, given time. On the other hand, there are some authors I used to worship and now I can't stand (as a forum staff member, I'm supposed to be impartial so I won't name names here). Right now, for example, I'm deeply entangled in the study of a belief system that I completely misunderstood and accordingly dismissed twenty years ago.
Second: Understanding something is still a far cry off from being able to use, control, or change it. Furthermore, it's beneficial to accept that not everything is knowable. If a doctrine say that Eyn Sof, the All, Nirvana, etc. is ineffable, we should be prepared to draw the line here - transcendence is transcendence, no progress possible beyond this point. Sorry guys, no peeking under the skirts of the universe.
Third: If your belief paradigm(s) recognises the existence of a higher self or a guardian angel, beware of narcissism. It's all too easy to inflate your faulty limited ego into an idealised version of yourself, only with superpowers and infinite wisdom that is accidentally in perfect harmony with all your present convictions. On the contrary, such a being (or psychological construct) should be far beyond everything you are and what you know at your present stage of development.
Four: If a book is fascinating, it's usually important (unless it's fiction or read purely for entertainment purposes). Of course, you can never be 100% sure but it's a good rule of thumb, in my opinion. If a subject really grips you, be grateful, for it's a precious gift from fate.
Five: Daily empty-mind meditation, not only on the pillow. Keeping your head free of chatter, ruminations, worries, etc. at all times will work wonders for your overall concentration faculty.
Six: Especially beware of the power of words and narratives! Examples and analogies, as apt as they may be, can only ever illustrate a point but never prove it. Particularly so-called 'holy' books of venerable age have a way of enmeshing you in myths, memorable quotes, gripping stories, etc. that it's hard to keep an open mind and some healthy sceptical distance. If something rings true, it's not necessarily the genuine article, it may simply mean that the author is a very talented writer.
I could probably think of more but that's probably it, for the time being.