Why past perfect in the following?

The Past Perfect is used to show that the action of signing had been completed before the next action took place. In this particular example, however, I would find it better if the next verb, took, were in the same form, since both actions were completed before the next one, i.e. the lawyer's gloating: After the rancher had signed the release and taken the check, the young lawyer...

The Simple Past in the same construct would not be wrong either, since the same meaning is conveyed with the use of after: After the rancher signed the release and took the check, the young lawyer couldn't resist gloating a little over his success,...

It is a matter of whether you want to emphasise the fact that the first actions had been completed.

The whole story is set in the past, before the time it’s being told. Various things happened: the attorney cornered the rancher, the lawyer did his best selling job, the rancher came to a compromise. After all that, the lawyer spoke to the rancher. The significance of the timing is that whatever the lawyer said, it was too late for the rancher to do anything about it. The past perfect construction emphasises the point that the rancher agreed to take half of what he was asking before the lawyer revealed what seems to have been his duplicity. The writer could have used the past tense signed instead, but the past perfect construction shows how the lawyer was careful to make sure the rancher did what he wanted him to do first.

I agree with Irene that consistency really requires taken.