'Letter & spirit' is an idiomatic antithesis of English language. Both are written in the same sentence but both have different meanings.  

By 'letter' we mean the literal interpretation of the data- what is written in words. And by 'spirit' we mean the intention of that data, what it is trying to convey to us. In other words, 'letter' is a common noun while 'spirit' is an abstract noun.

• Sometimes we see the usage of this idiom in a sentence like; The scheme will be followed in letter & spirit. It simply means that they will not only follow what is written in the scheme, but will also adhere to its intention. 
• While at other instances it could be used as a contradictory form; He went on to follow the Constitution's ideas in letter but not in spirit. This means that he adhered to what was written and stated the facts of the Constitution but did not apply its intent in his actions.