For integral forms, I like Rational Quadratic Forms by J. W. S. Cassels, it is available as a Dover reprint. But some of your choice of language suggests you might prefer Conway and Sloane, Sphere Packings, Lattices, and Groups. I get useful results from Integral Quadratic Forms by G. L. Watson. Any of these is easier than O'Meara. But perhaps you will learn all you need from Serre and Milnor and Husemoller, as mentioned above.

For integral forms, I like Rational Quadratic Forms by J. W. S. Cassels, it is available as a Dover reprint. But some of your choice of language suggests you might prefer Conway and Sloane, Sphere Packings, Lattices, and Groups. I get useful results from Integral Quadratic Forms by G. L. Watson. Then there is The Arithmetic Theory of Quadratic Forms by B. W. Jones. Any of these is easier than O'Meara. But perhaps you will learn all you need from Serre and Milnor and Husemoller, as mentioned above.