Assalamu'alaykum.

There are many books on Islamic finance, but the best that I have came across for those who wish to understand the basic Islamic financial system are :

1. Islamic Financial System: Operations & Operations, ISRA (International Shariah Research Academy) - written by many scholars, academicians as well as practitioners, including experiences from central bank.

2. Understanding Islamic Finance by Muhammad Ayub (Wiley)

3. A book written by Dr Muhammad Imran Asraf Usmani (apologies can't recall the title)

Allahuallam. Wassalam.