Question: Can I write about a company and my experiences since I retired been with the company 21 years?Answer:
I am not a lawyer and cannot give legal advice. However, my understanding is that as long as you're being truthful, you are free to write about your experience, just like a journalist. (I'm assuming you want to write nonfiction, autobiography, or memoir?)
That said, if you are going to make negative or controversial claims about a company that could damage its business, it would be a good idea to look for additional evidence or testimony to corroborate your account.
The same is true if you plan to say things about other, real people that could damage their careers. In this case, you have the same obligation as a journalist to check your facts. On the other hand, if you want to change the names to protect the guilty, you have a little more freedom.
Once you have a draft completed, you and/or your publisher will probably want to get legal advice before publishing the book.