I was checking the relatedness demo and service, and realized that common words like book are unknown to the system, which limits largely the usefulness of this service for practical purposes.
It is also striking when compared with the public evaluations in http://www.cs.waikato.ac.nz/~ihw/papers/08-DM-IHW-Semantic_relatedness.pdf. I tried all pairs in the 353 dataset, and only half of the pairs had at least one word which was unknown to wikipedia.
Maybe I'm missing some parameter?
I too am experiencing this issue. Are there any updates?