I believe a professional reference letter should include: date of the letter, how long the writer has known the applicant, the writer's status in the business, background on the writer's working relationship with the applicant, positive endorsement of the applicant - both professionally and as the future owner/tenant in the co-op, good character examples of the applicant, and contact information of the writer.

I'm sure more or less can be included, but this only meant to be a general guideline. You may also wish to review online resources for specific examples.

Hope this helps