What exactly is a letter of recommendation?
A letter endorsing you for the goal to which you are applying. Quite frequently this is for a job opportunity, but it can also be for visas, citizenship applications, appointments to exclusive organizations or controlling boards, or other exclusive opportunities.
When asking for, or writing one, it's important to know why it's being written, because a good recommendation letter is a compelling endorsement to select the given individual for the given opportunity.
What value do these letters have for a hiring manager?
When the individual and the recommendation writer are unknown to the hiring manager, the letter gives a context about what the individual has done well and what kind of a good hire the individual will be. It also shows that there was someone out there willing to put their good name on referring the candidate.
When the writer or something about the individual's career history are known the hiring manager, it counts for even more. Some industries (even global ones) are small and quite well networked. The farther I get in my career, the less surprised I am when I find out that the people hiring me know the people writing my references. This counts for even more, since the hiring manager now knows how the reference thinks and what his or her basis of judgement may be.
Should I make a habit of asking managers to write me one when I resign?
Depends on your industry.
As an American engineering manager, I ask and expect to be asked regularly about my willingness to write reference letters. It's good to line up positive referrals when your daily interactions may be at an end. Resignation is a good time to exchange contact info with those you want to keep in touch with - references are just a part of that.
As an academic in the US, I found out that a friend of mine, instead, expects to collect reference letters for her CV at the time that she leaves a job. The general intention is that she is aiming to other institutions and that because her term of service in academia is fixed, she's looking for letters towards the very obvious next step up the ladder, so references know what kind of letter they need to write and what the acceptance criteria on the next option will be.
The less obvious the career transitions in your industry are- the more likely it is that you want people who are willing to be references instead of the actual letter.
Can I write my own letter and have someone else sign it?
In some circumstances (such as applying for visas and citizenship) the asker may provide a written letter subject to the review of the referrer. The times I've seen this, it's because the goal was very specific and certain criteria that may not be known to the reference MUST be part of the letter, or the opportunity is at risk.
In circumstances that are less strict (for example, job applications) referrers expect to be at liberty to write their own letters, they just want to know the criteria they are supposed to be writing for - for most of my job references, I've provided job description, my current resume/CV, and notes taken from my research on the position to the reference writer.
In all cases, referrers expect to be able to edit or tweak the writing. No reference is likely to be willing to sign virtually anything you give them, so if you give a letter, offer it in electronic, editable format and plan time for the reference to review and edit it.
Do letters from managers or clients have the same value as those written by colleagues or family and friends?
For the most part, the letters from those who have very little bias and who have had to receive the direct value of your work (managers and clients) hold more weight. Also people of more senior positions, or highly respected skills in the industry hold more weight as experts on personnel.
Family and friends are useful, but may be assumed to be biased.
The degree to which these groups differ has a lot to do with the nature of what you are applying for. Things that are focused on personal character may rate managers, clients, colleagues, friends and family almost the same. Opportunities that require your career skills are going to consider people who have been associated with you and dependant on you in your career more strongly.
Some positions will draw clear lines (1 reference at most from friends & family, at LEAST 1 reference from a former manager, etc) so having a diverse collection of willing references is ideal.
However, it's understood that people the beginning of their career don't have that many bosses who can give a reference... having not had many/any bosses before.
Are these letters used differently in the public or private sector or in academia?
And they carry different weight in different companies within a sector, and with different hiring managers within a company.
In all cases, you'll do better if:
- The reference speaks eloquently to the needs of the position
- The reference is honestly given and positive
- The reference is compelling in that the person giving the reference is trusted to be an adequate relationship per the requirements of the position application and more so if the reference giver is known and/or respected.