It is likely that the judge will read them. They are usually submitted to the court prior to sentencing. I suggest you contact the defendant's attorney and discuss the matter with him/her and have the letters submitted to the court through him/her or whatever other channel s/he would advise.
The lawyer should give you details on what the lawyer wants included. The letters will be used at sentencing which means that the lawyer will not want people to write that the defendant would never do whatever it is s/he has pled guilty to.
Generally, letters should detail how the person knows the defendant, how often they are together, good things that the person knows about the defendant - helps little old ladies across the street, takes meals to shut-ins, etc. The lawyer may want the letters to discuss the difference in the defendant before and after an addiction, if applicable, and the improvement since arrest, etc. Some times it is good if the defendant has apologized to friends and family for embarrassment, etc.
You really need a LOT more direction on what the letters should include. The lawyer should compose a letter with the suggested content, depending, of course, on the writer's knowledge.
All letters should include complete contact information and be typewritten and legible.
The judges DO read the letters.
It is very common in federal court for the defense lawyer to submit a sentencing memorandum to the judge together with letters from family and friends about the defendant. The letters are used at the sentencing proceeding. Federal judges take sentencing very seriously and will read everything submitted to them. The purpose of the letters are to show a side of the defendant other than the fact that he committed this crime. The judge will also receive a presentence investigation report about the defendant from the Probation Office, but that tends to focus more on the negative. The best letters are ones that explain how the writer knows the defendant and what the defendant is like as a person with specific examples. It is always best if the information allows the judge to decide what sort of person the defendant is rather than just conclusory adjectives. Instead of saying the defendant is kind, for example, talk about how he mows the lawn for his elderly next door neighbor. Being a loving father and husband, belonging to church groups, running a business, etc. are all relevant and important. This is an attempt to humanize the defendant. For more info, speak to the defendant's lawyer.