There is some debate about the value of a letter of reference. My take is that there is some value but it depends on who it is from and what it is that they say.
A reference letter from a former boss may or may not be valuable. You need to consider the possible reasons for the letter and judge it in context.
Many years ago, I worked with an engineer who was quite talented. He was also loud, opinionated, difficult and all those other qualities that make it hard to be around a person. It does not mean that we did not value his opinion but he was a real pain.
After about 2 years working with this fellow, he announces that he and his family are moving to another state. Now my boss had always had a rough time with this fellow and they often butted heads. There was genuine dislike between the two.
When my boss heard he was leaving, he wrote the most glowing recommendation for this engineer I had ever heard. Why? Because he was going to be out of his hair and he was going to torturing someone else and that was their problem. See where I'm going here. There was not one lie in that letter. It talked glowingly about the engineering skills, depth of knowledge and ability to solve complex problems. All true, but there was not one word about opinionated, grating or obnoxious.
So the question is what do you look for in a letter of reference. Here are some good examples and ideas to consider.
A reference letter that speaks about specific accomplishments that were above and beyond the call are valuable. Also if the letter mentions specific awards or achievements that is valuable. Additionally specific tasks that are called out for merit are helpful but the key is specificity.
Sometimes a candidate does not like to brag about themselves. In such cases, a letter of recommendation with facts and figures about how much things were improved or how much money was saved by the candidate are helpful in evaluating a candidate.
Letters of reference are often a kind of currency for people who do project work. In these cases they are brought in short term to fix a specific problem then let go. Other than making some money there is no real reward for the work that is done. If a colleague or supervisor to write something for the work done in this project it is valuable in understanding the contributions a candidate can make in a short term assignment. This suggests they can come up to speed quickly and contribute quickly.