Without having read the book in question and examining the context of the statement, I cannot comment on what the author meant. All I can say is that eating between meals is not inherently sinful. It can be an occasion of sin, allowing for the intemperate use of food, but it is not sinful in and of itself. If one has issues with food and is trying to eliminate occasions for the possibility of gluttonous eating, eliminating snacks between meals might be a good place to start. But a snack for someone else, who does not have problems with food, would be perfectly fine.
