First 100% is hard to get especially on big projects ! and even if you do when a block of code is covered it doesn't mean that it is doing what it is supposed to unless your test are asserting every possible input and output (which is Almost impossible).
So i wouldn't consider a piece of software to be good simply because it has 100% code coverage but code coverage still a good thing to have.
Well, isn't it much more easier to go directly to the third step, and do never make BooksLimit() unit test at all?
well having that test there makes you pretty confident that if someone changes the code and the test fails you will notice that something is wrong with the new code therefore you avoid any potencial bug in your application
