Here is something new I learned. While making a map-book, its irritating to see all the legend items added in your legend. I learned how to quickly add legend for feature that is displayed in the Page of Data driven page.
I assume that readers are familiar with Data Driven Page. Once data driven page is enabled on a feature layer, legend can be added in the usual manner;
Legend will appear like this;
Right click on legend added in ArcMap, go to properties;
In properties, navigate to Items Tab. There are two checkboxes named 'Only show classes that are visible in the current map extent' and 'Use current map feature as map extent', check them both;
And we are done!! Now you can see only legend description of feature which is in data frame and it will change as you scroll through you data driven pages;
I am attaching a mapbook sample that I created using dynamic legend, forgive me for shabby placement of legend box.
I hope that was helpful!