GST is still evolving as a law and you can refer to Model Law published by ICAI.

This is available as free download from idtc @ icai.org

we have to wait ., we can know after 2017 

The best book on GST IS

KEY FEATURES OF THE BOOK ♦Detailed commentary on Model GST Law to encapsulate the provisions for easy digest ♦ Overview of present indirect taxes-shortcomings, cascading and double taxes ♦ GST-need and necessity, what it is, how it works, etc., alongwith discussion on challenges ahead ♦ Detailed discussion on meaning and scope of the term 'supply' alongwith the discussion on likely issues ♦ Detailed discussion on meaning of taxable persons, liability to be registered, exclusion and compulsory registration ♦ Discussion on various domains viz. inter-state/intra-state supply of goods and/or services, principles of place of supply & time of supply, exclusion and exemptions in GST, exports & imports under GST, determination of Revenue Neutral Rate (RNR) etc. in a pragmatic manner through illustrations for easy digest ♦ Discussion on flow of input tax credit in GST with multiple illustrations ♦ Business processes and procedural aspects of GST-registration, return, payment, refund, demand & recovery, adjudication, appeals, etc., alongwith key suggestions ♦ Journal entries for various scenarios in GST to provide clarity from accounting perspective ♦ Transitional issues and impact of GST on manufacturer, trader, service sector and specifically on various sectors viz. alcohol, real estate, automobiles, telecom, FMCG, insurance, e-commerce etc. ♦ Section-by-section analyses of the Constitution (101st Amendment) Act, 2016 and its comparison with the 122nd Constitutional Amendment Bill, 2014 ♦ Journey of GST including important milestones with way forward for GST ♦ Comprehensive FAQs to provide conceptual clarity on various aspects of GST in easy and understandable way ♦ Appendices on Model GST Law, 2016, Constitution (101st Amendment) Act, 2016, GST Draft Rules & Formats on registration, invoice, payment, returns and refunds, FAQs released by CBEC etc.

SEE THE BOOK YOU WILL REALLY FIND IT GOOD

https://www.amazon.in/Analysis-Conceptual-Procedures-October-2016/dp/8188274658/ref=sr_1_3?ie=UTF8&qid=1479314942&sr=8-3&keywords=gst