Amazing Spider-Man #422
This is one of the best issues to read if you are learning about Electro. It is the best Origin issue of Max Dillon's life. He also gets a huge power-up in the series.
Amazing Spider-Man #612-614
This is part of the gauntlet series in the Amazing Spider-Man Series. Electro gets revamped in this three book series and levels up as a villain.
Amazing Spider-Man #9
The first appearance of Max Dillon/Electro.