How do we use RMA module of Magento?
Is there any book or manual to get hang of Magento?
There is good source available for learn magento basics.You can start from good source available by magento : http://docs.magento.com/m1/ce/user_guide/Magento_Community_Edition_User_Guide.html. And Technical document : https://magento.com/resources/technical And MagentoU offer some free and paid course and video tutorials :https://u.magento.com/
You can also find some good blogs by other website. As you mention for RMA you can see some good post here : http://inchoo.net/magento/magento-enterprise-return-management-authorization-rma/
And on official site :http://docs.magento.com/m1/ee/user_guide/order-processing/rma-workflow.html
Hope it will help.