What constitutes virtue and how to train myself in virtue? It is said that Virtue is the ultimate weapon. It is also said that virtue automatically leads to enlightenment without any volition or desire to achieve enlightenment. 
Therefore I want to develop 'heightened' virtue but I am unable to find any sutta which explains what constitutes Virtue.

In the following quote from AN 3.90 training for heightened virtue is mentioned:


  “And what is the training in heightened virtue? There is the case
  where a monk is virtuous. He dwells restrained in accordance with the
  Patimokkha, consummate in his behavior &amp; sphere of activity. He trains
  himself, having undertaken the training rules, seeing danger in the
  slightest fault. This is called the training in heightened virtue.


My questions are :
1. What constitutes virtue ?(Any reference to Suttas would be great.If there are any books giving short summary of Patimokkha in English then that would also be great.)
2. How to train myself in virtue ? (Are there any training books available?)
Virtue or sila at the very minimum is the five precepts. Further practice of virtue is by the eight precepts, ten precepts, Right Livelihood, Right Speech and Right Action.

Further information can be found in AccessToInsight's virtue (sila) page, including sutta verses.

To practice virtue (sila), the first step is to vow to undertake the training of the five precepts, and then practice them with heedfulness (appamada).

The second step is to adopt this thinking from AN 5.57: 


  “And for the sake of what benefit should a woman or a man, a
  householder or one gone forth, often reflect thus: ‘I am the owner of
  my kamma, the heir of my kamma; I have kamma as my origin, kamma as my
  relative, kamma as my resort; I will be the heir of whatever kamma,
  good or bad, that I do’? People engage in misconduct by body, speech,
  and mind. But when one often reflects upon this theme, such misconduct
  is either completely abandoned or diminished. It is for the sake of
  this benefit that a woman or a man, a householder or one gone forth,
  should often reflect thus: ‘I am the owner of my kamma, the heir of my
  kamma; I have kamma as my origin, kamma as my relative, kamma as my
  resort; I will be the heir of whatever kamma, good or bad, that I do.’


The third step is to understand more deeply from the Dhamma on Right Livelihood, Right Speech and Right Action, and practise them with heedfulness (appamada).