Q:

Why is this not an isomer of pentane

Given was the fact there are 3 isomers of pentane, two were given: pentane and 2-methyl butane. Then I was asked to draw to other one. I drew up the star like structure and checked it to be ok (coincidental I might add).
Then I drew some more and came up with amore turnstile shaped molecule. 
     H
     | 
   H-C-H   H   H
     |     |   |
   H-C-----C---C-H
     |     |   |
   H-C-H   H   H
     |
     H

To me this also seems like $\ce{C5H12}$ molecule and all valences seem ok. Why isn't this a valid isomer?
Edit: this is 2-methyl butane according to the book:
        H
        | 
   H  H-C-H   H   H
   |    |     |   |
H--C----C-----C---C-H
   |    |     |   |
   H    H     H   H

A:

To clarify Apeiron's answer. You have drawn 2-methyl butane, the same isomer that was given in the question. The rotation of a bond is probably the source of any confusion. A 3D model of your two structures would be the same.
To answer your question:
Neopentane is the third isomer of the pentane series.

