This amazing classic can be downloaded for free (or for a small donation of $2.95 for a PDF version) from http://ccel.org Classic Christian Ethereal Library. This is a great site as iit has many public domain books that are classics in Christian Literature. I have been using this site for years, and have found many of the great classics there.
The Interior Castle by St. Theresa of Avila seems as if it were written yesterday, instead of 1583. Her metaphors and descriptions of one’s various stages of spiritual capacity (mansions) are well done, and brilliantly illustrate the spiritual light and passion to which she is so well attuned.
Please pick this book up for free or download it for free (or PDF). If you have a nook, it can be downloaded to your My B&N folder on your computer from http://www.ccel.org and next time you update your nook, you can copy it over. OR you can download a free e-reader from Barnes and Noble and read the text version which is free.
This book is worth your time and investment of it, much the same as The Cloud of Unknowing. It is one of the mystical classics.