what is the connection between sithis and alteration? when u read the book sithis u get alteration skill, is this just some random thing or is alteration sithis' favored school of magic? or what?

Schools are artificial constructs, so I doubt half the the galactic interplay is able to have much of an opinion on it. Or for that matter, hold any sort of opinion at all.

Rather, reading the book gives you some insight into the nature of the world. Understanding this nature makes it easy to alter and bends its laws. Hence its benefits to Alteration.

well, since sithis is the personification of change, then reading his book and learning alteration would make sense as it's really a magical art of change (change and alter are synonymous)