...who happened to do the art on the stories reprinted within....
...and here's a little on those stories all about the robotic foe of the Avengers!
The Early Days of Ultron
Ultron premiered in Avengers #54 (July, 1968), but wasn't seen as Ultron until Avengers #55 (August, 1968), working with the Masters of Evil, but this issue collects Avengers #57 (October, 1968) and Avengers #58 (November, 1968; all issues by Roy Thomas and John Buscema). Within these pages, the Vision is introduced, tries to join the Avengers, and kill them as well. Vision revolts against Ultron's control, and along the way, we learn the origin of Ultron, how he was created by Hank Pym (Ant-Man/Giant-Man/Goliath...and more?).
Ultron Returns Again
Ultron did return multiple times, most notably in Avengers #162 (August, 1977), where he created Jocasta, who was meant to be his bride, and the second selection of stories in this collection are Avengers #170 (April, 1978) and Avengers #171 (May, 1978; all these stories by Jim Shooter and George Perez). Ultron, like Frankenstein's Monster, just wanted someone to share his lonely existence with, and tried to create his own mate, Jocasta. This didn't work out so well for Ultron, as he was stopped by the Scarlet Witch and her unpredictable mutant hex power.
Even More Than Ultron?
If you are unable to find the Avengers: Ultron Unleashed, and don't quite want to spend money on getting the actual issues, you can also find the earlier issues in Marvel Masterworks: The Avengers #6 (reprinting Avengers #51-58, Avengers Annual #2 and X-Men #45...a whole wonder of great stories, including Ultron's first appearance, the second Masters of Evil and a case of the Avengers vs. the X-Men!).
The second batch of issues are included in Avengers: The Korvac Saga, which includes Avengers #167, #168, and #170-177, which include folks like the original futuristic Guardians of the Galaxy, the Collector, the Kree Captain Marvel and more Avengers than you can shake a stick at as the team fights for universe against Korvac.