[Kuruharan]

That is a very good question that I had never really thought of before!

I think that Shelob was probably more than just a beast. Her mother was a spirit of power and some of that probably came down to her daughter. She likely had at least some degree of intelligence and 'reason,' maybe even a fairly high order of those qualities. Just because she was primarily interested in food does not mean that she is unintelligent. Like Sauron, who would hardly be called dumb, she focused all of her energies into eating everything in sight, like he focused all his power into dominating everything in sight. (Not to say that they were in any way equal, just a comparison.)

As for the means of communication I really have no idea. There may have been some sort of convergence between the two of them because Shelob could probably tell that Gollum was a creature of more than usual malice, who might be useful in finding more food (since he was rather "fleshless").