Excellent question!  Books are the perfect gift--they are always the right size and they never go out of style. And some of the books your granddaughter reads now will no doubt have an important influence on her as she gets older.
 
The appropriate grade-level or age listed for a book is determined by a number of factors.  There are some readability tests that factor in the complexity of the vocabulary and sentence structure.  There is also the more subjective issue of maturity and readiness for certain concepts and themes.  And there is the important question of a child's interest in certain subjects.  Finally, there are also topics that are usually covered at each grade level.  Reading a book that touches upon these topics will deepen a student's understanding.
 
Usually an age or grade range, rather than a specific age or grade, is listed. For example, most books at this level are considered appropriate for ages 6-8 or Grades 1-3.
 
The perfect book for a child is one that he or she will enjoy reading, one that will challenge without frustrating the child, and one that the parents feel is appropriate for the child's age.
 
You could ask the parents what books your granddaughter has recently enjoyed and use those as guidelines.  Amazon.com even has a function that suggests books based on ones you have enjoyed in the past and will help you find books for the appropriate age range and in a theme of interest.
 
Here's a guide to choosing good books:
http://www.education.com/reference/article/Ref_Choosing_Good_Books/
 
View any lists as recommendations based on guidelines, rather than as prescribed requirements.
 
What I would recommend is to take into account your granddaughter's age (is she older or younger for her grade), comfort with reading, and maturity level.  Most likely if she will be attending second grade in a couple of months, you would select from the second grade list.  However, if you feel there is a book she would love from the first grade list, or even the third grade list, she will most likely enjoy the story.  Even better, perhaps there is a book in a shared area of interest so you can talk about the story with your granddaughter.
 
Whichever book you choose, you are giving a great gift--a treasure you may enjoy and discuss with your grandchild!
 
 
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Candace Lindemann, Ed.M. - JustAsk Expert
Educational Consultant and Writer
http://CandaceLindemann.com