Charles Webster Leadbeater published A Textbook on Theosophy in 1912. This book describes the fundamentals of Theosophy to anyone who is interested in the topic.
For a book written in 1912, where florid styles were all too common and readability was thoroughly secondary concern, this textbook reads remarkably clear. By an large, the writer shows restraint, indicating that the author truly did intend this as a textbook, its purpose to explain Theosophy clearly. So if you want to learn something of Theosophy, or the Theosopohy of 1912, you would do well to start with this book. Myself, I knew almost nothing of Theosophy, and after reading this book, can talk at length about the subject. So as a textbook, I think that this work proved rather successful.
Many parts of the book read archaically. As the technical terms of our language have changed, so too has the apparent meaning in the book. In places, the author refers to "vibrations", making his analogies sound rather strange, Today we use the word "frequency" to describe differing vibrations. For example, instead of two worlds existing at two different vibrations, which sounds like a cheap 1920's planetary romance sort of thing to say, two planes would exist at two different frequencies, sounding something more like modern SF. Thus, although the language often sounds antiquated, the notions themselves persist in our culture in different guises.
If you are already familiar with eastern philosophy, many parts of this book will seem simplistic and rudimentary. In the world of 1912, concepts from the east, such as karma and reincarnation, were little known in the west, and so require significantly more explanation to make sense of. Today, the explanations would still have been there, but the actual word would have been used rather than longer-winded explanations.
In many places, the use of scientific words is not only inapplicable, but completely misapplied. For example, Theosophy is often called a science, with its ideas being scientific. In no sense of science, even in the 19th century, would this apply. Knowledge given to us never counts as scientific.
The book makes several references to spells, Masons, and other aspects of the occult, the hidden lore of a world, but I cannot describe this as a book of occult lore in the modern sense. You will be no more capable of doing anything amazing after reading this book than you would be before reading this book, except for finding inspiration and reason to make you a more generous humanitarian. Somehow making yourself a truly unselfish person, acting in the best interest of every, is amazing enough for any book.