When I am reading Serre's $A$ $Course$ $In$ $Arithmetic$, Chapter 5, it deals with $quadratic$ $forms$ of some vector space $V$, which can be viewed as an extension of an $abelian$ $group$ $E$ of rank $n$.

And it talks about some $invariants$ on E, such as discriminant, $\varepsilon=\prod(a_i,a_j)$. In particular with discriminant 1.

I would like to read some reference to handle it more fluently, since I can't handle some properties in the book, for examples:

https://math.stackexchange.com/questions/1127475/two-modules-with-the-same-index-have-the-same-discriminant

and

(It suggests J. CASSELS, Comm. Math. Helv., 37, but I can't find that file.)