Why did he agree to work with the book's author in the first place?

He wasn't at all certain she would agree to work with him.

From then on the two agreed to work together, having a financial interest in each other's success.

Everyone had agreed to work according to the same plan.

After you've agreed to work on a project, how do you begin the design process?

In light of this, they agree to work together to track him down.

Of course an agent in my office will agree to work with you.

"I thought we had agreed to work on that thing together."

Then, in the late 80's, the companies agreed to work together on personal computer operating systems.

A total of twelve men agreed to work together to form the bank.