According to camelcamelcamel.com, its price spiked recently. It was more like $30-35 for a while (including when I bought my copy). It is really good, describi

According to camelcamelcamel.com, its price spiked recently. It was more like $30-35 for a while (including when I bought my copy). It is really good, describing several different architectures for computation. Different designs for virtual machines, if you will. It isn't limited to Lisp, either -- Prolog and a couple other language families are covered.