How to interpret this IC diagram?
That notation is indicating the "Enable" pin expressed by "E" is active low. Meaning to enable each decoder, you must pull the "Enable" pin low.  Check out the pin descriptions on page 3 of the datasheet here.  This type of notation is common.  You will see this type of thing implemented in several ways across different IC manufacturers.  i.e. 
\$\text{Enable}\$, 
\$\overline{\text {Enable}} \$, 
\$\text{SD}\$ (Shutdown), and 
\$\overline{\text{SD}}\$ (barShutdown). 
You just have to the reason through the language.  \$\text{Enable}\$ is active high enable, 
\$\overline{\text{Enable}}\$ is active low enable, 
\$\text{SD}\$ is active high shutdown, 
\$\overline{\text{SD}}\$ is active low shutdown.