Writing generally "reusable code", aka a library, warrants a different approach to software development than application code in many areas.
1. Application code = Fast-changing, poorly specified code. You need to have a rapid development cycle that supports "discovering" what the customer wants along the way. Your #1 job is pleasing the customer, as quickly, and as reliably, as possible.
2. Library code = Slow-changing, highly specified code. You have a long, conservative development cycle. Your #1 job is supporting application programmers (the customers of your library).
1. Application code = Fast-changing, poorly specified code. You need to have a rapid development cycle that supports "discovering" what the customer wants along the way. Your #1 job is pleasing the customer, as quickly, and as reliably, as possible.
2. Library code = Slow-changing, highly specified code. You have a long, conservative development cycle. Your #1 job is supporting application programmers (the customers of your library).