Yes, "tontos" is not a word you would tell to somebody you want to sell a book :)
Instead, I would use:
... para principiantes
principianteis a word you can find in the DRAE meaning "beginner":
(Del ant. part. act. de principiar).
adj. Que principia. U. m. c. s.
adj. Que empieza a estudiar, aprender o ejercer un oficio, arte, facultad o profesión. U. m. c. s.
However, this collection of books has been published in Spanish with the "para dummies" text in their title. See the article in Wikipedia about them: Para dummies.
It seems that the tontos, idiotas debate is vivid in comments. Nice! To add to the debate, I recall reading some books para torpes, that had a funny approach together with images by the famous cartoonist Antonio Fraguas, Forges. You can see the collection in: "Libros para torpes".
This torpes has a funny connotation as somebody who is not very skilled, even though he/she tries hard. In the early days of Internet, these kind of books were very successful as they would explain the basic concepts in a very easy way, so people could catch up the brand new technology.