The Secret Doctrine is a very long book, and there is also the 12 volume Collected Writings of Madame Blavatsky.
Then there are other writings from buddhism, hinduism, sufi, zen and other traditional sources which can also be found in theosophical libraries, as well as astrological, Rosicrucian and other books too.
There is a lot to choose from, and I have selected a few that I think are most relevant and useful
- The Constitution of Man
- The Structure of Reality
- The Masters of the Wisdom
- The Mahatma Letters
- Schools of Yoga
If you are intereested to know more, the Offshoots of Theosophy page contains all the links