This is an excellent novel that is all about the transforming power of love. Thinking about the end of the novel, note the way in which after Leslie's death, Jess's parents rouse themselves and show their son the love and affection that Jess needs at this stage in his life. It is this love that in turn transforms Jess and enables him to conquer his fear of drowning so that he can save his little sister. The way in which the resolution of the novel shows that Jess has dealt with Leslie's death and moved on is the way in which the novel ends with Jess building a bridge to Terabithia for May Belle, so that she can participate in the joys of Terabithia and value her own life in the same way that Leslie helped Jess to value himself:
And when he finished, he put flowers in her hair and led her across the bridge--the great bridge into Terabithia--which might look to someone with no magic in him like a few planks across a nearby dry gully.
The power of the imagination and what it can do for our lives and transform our own view of ourselves is thus metaphorically passed on to May Belle as she is initiated into the kindgom of Terabithia and becomes its new queen.