It is said that many intelligent and otherwise capable people owned the Secret Doctrine but only a handful actually understood and comprehended it in its totality.
The problem is that the esteemed author, Madame Blavatsky used a prose and style that many contemporary readers would have trouble reading and understanding. The sheer size of the books is formidable.
In addition the Secret Doctrine attempts to cover everything from the complete Cosmology of The Universe, God and the devil - just to name a few of the subjects. The third, also called Lost Volume of the work was published and mostly written by her disciple Annie Bessant, who naturally knew a great deal less than her Master.
According to P.G.B. Bowen, Blavatsky gave the following instructions regarding the study of the Secret Doctrine:
"Reading the Secret .Doctrine page by page as one reads any other book (she says) will only end us in confusion. The first thing to do, even if it takes years, is to get some grasp of the "Three Fundamental Principles" given in the Proem. Follow that up by study of the Recapitulation - the numbered items in the Summing Up to Vol. I (Part 1.) Then take the Preliminary Notes (Vol. II) and the Conclusion (Vol. II)."
Reading this guide book and appendix A and B will certainly help understanding the most important book in the last two hundred years.