Well, many students simply won't want to learn more math during the summer, the chance that students will be more distracted is higher.
You have to consider who it will be beneficial to before considering if it will be beneficial. A student who needs the extra practice for memory may want to use the books. A student who is failing or struggling may also want to use the books.
Just throw the books at them and expect them to learn? Of course not! I personally recommend doing math-related activities, possibly show more applications of math. You may find many applications of math here.
Personally, as a student, books? +Math? During summer? You can see what I'm thinking, can't you? A student doesn't like to study through books, most students don't want to study math, and studying during the summer is generally a child's nightmare! Make it fun, that's the key.