I'm currently reading "Claiming Knowledge. Claiming Knowledge. Strategies of Epistemology from Theosophy to the New Age" by Olav Hammer which comes in at a whopping 569 pages. It's a scholarly work spanning multiple schools of esoteric thinking, so it's understandable that so many pages are needed in the interest of comprehensiveness. What's more, it's extremely scholarly so a lot of space is needed for footnotes and quotes, also to thoroughly underpin the author's claims and theories that would go unsubstantiated in 'technical' occult books. Bearing in mind that Blavatsky's and also Steiner's tomes are full of the wildest assertions and that the author not only tries to trace them back to genuine Indian, Egyptian, etc. sources but also analyses how they have influenced contemporary New Age thinking, it's small wonder why that book is so voluminous. I love how the author is neither hostile towards occultism nor attempts to actively debunk theosophical or anthrosophical doctrines, he simply demonstrates the mechanisms by which those esoteric teachings were generated, and of course you need a lot of material as a base of such scholarly analyses. It's eye-opening, to say the least.
Now and again I would return to Lars Helvete's "Belief Magic - Belief as a Means of Magical Power" with its 18 pages, more a treatise than a book, and try to wrap my head around his thinking. It's a much more mature and philosophically sophisticated version of Peter J. Carroll's theories about the chaos magic, random belief and paradigm shifting, not dense at all and even couched in simple language; I nevertheless find it hard to get to grips with it. I am sure its implications are profound but I have yet to find out how they are important specifically for me.
What I hate is when a book contains nothing but fluff in the beginning, for example a lot spurious invented tradition and a highly selective reading of history to justify the author's ideas, rituals, etc. that he or she describes only much later his or her book. Sometimes all that fluff is crucial, for example in the GoM books where the soothing writing style of the opening chapters prepares the reader's mind for the practical instructions that come later in the book, but having read all those scholarly books about magic and its history, the term 'ancient wisdom' is forever ruined for me, and that's a good thing. When I read now, for example, "Already the Gnostics knew…", I can't help but thinking that the word 'Gnostics' is now mostly used in quotation marks by scholars because it's such an imprecise umbrella term for extremely disparate philosophical currents that it's as good as useless for the study of a history of religions and also for practical magic.
In short, I think it's about the subject of the book in question, whether it covers a wide range of topics or a single highly specific one. Ideally, the former type is expansive and the latter short and concise, but in recent years a lot of badly written 80-or-so page potboilers have been published by occult authors desparately striving to quit their day jobs, and I'd really hate to be tricked into paying good money for them.