Here you can find some lecture notes on various topics.
1. Invariant theory:
Michel Brion:
Invariants et covariants des groupes algebriques reductifs.
William Crawley-Boevey:
Lectures on representation theory and invariant theory.
Kraft, H. , Procesi, C.:
Classical Invariant Theory, a Primer.
2. Quiver Theory:
Jon Brundan:
Topics in Representation Theory.
William Crawley-Boevey:
Geometry of representations of algebras; Lectures on representations of quivers; Representations of quivers, preprojective algebras and deformations of quotient singularities.
Harm Derksen:
Quiver representations.
Lieven Le Bruyn:
Noncommutative Geometry @n.
Charles Walter:
Variation of Quotients and Etale Slices in Geometric Invariant Theory.