If a book is no longer under copyright, can I make copies of it? What if I bought it on the Kindle, or another e-book seller?

If the book is in fact no longer protected by copyright, then yes, you can make copies or do anything else that is otherwise protected by copyright law, like make derivative works of the book. That assumes that you are correct that the book is no longer under copyright protection. Any work that was created in the United States more than 75 years ago is no longer copyright protected. If it was created after that, there is a myriad of rules that determine whether the work is still protected. If the work was created less than 75 years ago or in a different country, you should consult someone with familiarity of the copyright term rules. Your follow up question is slightly different. If you bought an ebook of a book that is no longer copyright protected, then the content of the book is not protected but the file may be. They may have used DRM (Digital Rights Management) and you may run into a problem if you try to circumvent the DRM.