بسم الله الرحمن الرحيم

The book "Funds in the Khilaafah State" is one of the best books available in English on the subject of Zakat and other related subjects.  It has been posted on line at the following link:

http://www.hizb-ut-tahrir.org/english/books/pdfs/amwaal_funds_in_state.pdf

I strongly recommend reading this book in its entirety.