Persons who haven't invested too much into a profession are in a good position to switch careers - to a library job. This is not to say that a person who has been in a profession for a good number of years cannot or should not consider a career change. Often, it's the length of time in a profession that drives people out. Burnout, changes to the profession itself, changes in the ages of the kids, even physical requirements can cause a professional to seriously consider a different career. However, the higher the salary, the greater the income gap to consider if an entry level library job is calling you.
Some situations allow established professionals to ease into a new career.
With a spouse relocation situation, both partners are going to a new job. If income is not an issue, wonderful. Again, options vary according to a professional's unique situation. For persons who, for one reason or another, take a quick exit from their current job, the options are to take a little bit of time after leaving to look around, or take the best available job open at the time. No wasting time scanning job ads or second guessing.