India is a Sovereign Socialist Secular Democratic Republic (Preamble). It is a Union of States (Section 1). The supreme law is the Constitution of India which was adopted by the Constituent Assembly on 26 November 1949 and came into force on 26 January 1950.
The Constitution is federal in form and has the traditional characteristics of a federal system - supremacy of the Constitution, which enshrines the division of power between the Federal Government (known as 'the Centre' or 'Union Government') and State Governments, and the existence of an independent judiciary.
The Constitution recognises certain basic fundamental rights (Part III) and prescribes certain fundamental duties (Part IVA) for every citizen of India. Any infringement of these rights can be challenged in the Courts by any citizen of India.
The basic structure of the Constitution is unchangeable and the only allowable amendments are those that do not affect its basic structure or essential character. The Constitution has been amended frequently - to date (March 2020) there have been at least 104 amendments to the Constitution.
The Government of India Legislative Department website includes all Amendment Acts - including Statements of Objects and Reasons for each amendment. Note that this site may be a little out of date and not include the most recent amending Act.
The official Legislative Department Constitution website does not always have the most up to date consolidated Constitution, incorporating all amendments. Nor can you rely on any open-access databases to contain the most up to date consolidation.
To ensure you are viewing the most up-to-date consolidation (with Statements of Objects and Reasons), use Manupatra (UniMelb staff & student access):
The Constituent Assembly Debates (vols 1-12; 1946-1950) are available:
See the following books for commentary on the Constitution and the making of the Constitution:
Norms and Politics: Sir Benegal Narsing Rau in the Making of the Indian Constitution, 1935-50 by 
The Constitution of India: A Contextual Analysis by 
International Encylopedia of Laws: Constitutional Law: India by 
The Constitution of India: Incorporating all amendments up to and including the Constitution (One Hundredth Amendment) Act 2015 by 
Offend, Shock, or Disturb: Free Speech under the Indian Constitution by Commentary on the Constitution of India : an exhaustive article wise commentary on the Constitution of India based on plethora of case law by 