I am pretty sure that the dots are caused by the reproduction method used to print the books. They are in the book, so they are in the copies. So changing the DPI probably won't help get rid of them, though it will create a generally better copy. Generally you are always better off using a higher DPI, it just takes longer and uses more memory.

What you can do is photoshop them or get them amended digitally by someone who knows how. There are companies that do such clean ups, but they can charge quite a lot. You may be better off asking around techie types that might do you a favour. If you send them a copy of the file, they won't do any harm even if they fail to eliminate the dots.

You mention that the author is dead. I am not sure if you say this because of copyright concerns, but copyright lasts about 70 years after the death of the author. Additionally, the author may not own the copyright of the photos but either got the consent of the owner, or used the pictures with no permission obtained. Technically any photo is treated in the same way as a book, with copyright expiring 70 years or so (depending o country) after whoever took the photo has themselves expired. In practice most old pictures have no known photographer, but some companies won't touch pictures if there is doubt over copyright ownership.