what is A-Book and B-Book Broker in forex?

A-book brokers don't make the market instead they push your order straight to the liquidity provider in exchange for a commission. This is why A-book brokers charge near-zero spreads plus commission. The advantage of A-book brokers is more transparency and faster execution.

B-book brokers take the opposite side of your order and trade against you. The advantage of B-book is that you can use things like micro/cent account to execute small orders. Small orders are executed faster with B-book brokers bit they charge a wider spread.