Carolyn, you have asked a good question.  One easy way to tell the level of a book is to look at the back cover.  Often you will see the reading level is was written for.  Here is an example, you may see RL 1.9.  The RL stands for the Reading Level and the 1.9 represents first grade - ninth month.  Use this as a guide to help your child select books that are appropriate.  So for a second grader you can look for books in a range from 1.4 - 2.4.  Research has shown that it is OK, especially during the summer, to read easier books if that is what your child chooses.  Encourage him/her to read a wide variety within that range, but also cover different genres: fantasy, science fiction, fair tales, biography, animals, etc.

Schedule weekly trips to the library with your child.  They have a wide selection of children's books: series, nonfiction, & fiction, on his level.  Libraries also usually have magazine subscriptions that child love to review.  Check to see if they have a children's story hour.  That can be a great way to expose your child to different books and authors.  The media specialist/librarian can offer suggestions as well.

Just because your child can read, take time a few times a week to read to or with him/her.  You can be a great role model for him/her and it is a great way to spend time together.  After sharing a book, either a chapter or picture book, you can talk about the characters, problems, solutions, and outcome of the story.  Just as a note, picture books often have more sophisticated plots and vocabulary.  That's why they are better for your to do the reading.   Picture books can expand your child's vocabulary and expose them to complex situations that might not be in books on their level.

I know I went beyond your original question, but wanted to give you a few ideas of what you can do this summer with your child.  Happy reading!