GLSL 1.4 (GL 3.1).
Yes there have been GLSL 1.5, 3.3, and 4.0 since then, but none of these is a rewrite – just some extra goodies added.
GLSL 1.2->1.3 was the biggest leap for basic workhorse shaders (vertex and fragment), because it deprecated nearly all the legacy “built-in identifiers”. And that’s covered by the book.
Well i am pretty new at this but the opengl 3.2 shaders are the same that opengl 4.0 ones?
Yeah pretty much the same, if your talking the workhorse shaders (vertex and fragment).
Of course, GLSL 1.5 added geometry shaders, and GLSL 4.0 adds the tessellation shaders. But learn vertex and fragment shaders first and then the rest will make a lot more sense. Bottom line, the Orange book is still a good starting point.
Also don’t forget the OpenGL wiki and on-line GLSL tutorials.
What GPU do you have? You may not even care about some of these newer features yet if your GPU doesn’t support them.