The RSpec Book: Behaviour Driven Development with RSpec, Cucumber, and Friends (Facets of Ruby)
100% test coverage is a fantasy and a waste of time. Your tests should serve a purpose, typically to give you confidence that the code you wrote works. Not absolute confidence, but some amount of confidence. TDD should be a tool, not a restriction.
If it's not making your work come out better, why are you doing it? More importantly, if you fail to produce useful code and lose the contract, those tests weren't too useful after all were they? It's a balance, and it sounds like you're on the wrong side.
If you're new to Rails, you can get a small dose of its opinionated creator's view on testing in this 37signals blog article on the topic. Small rules of thumb, but maybe something to push you in a new direction on the subject.
There are also good references on improving your use of RSpec like betterspecs.org, The RSpec Book and Everyday Rails Testing with RSpec. Using it poorly can result in a lot of headache maintaining the specs.
Technically it's BDD, but I would recommend The RSpec Book because it does a good job of explaining concepts and has extended tutorials. The second section of the book covers Cucumber, so it will teach you that as well.
If you really want to research best practices for rspec some resources I use are: