I heard and experienced a little that unconscious mental processes involved in storing memories work really well when you are not thinking about your study subject during rest. However, I'm not sure mental effort such as reading a novel is as effective as physical effort for consolidation of studied information. What is the effect of irrelevant mental exercise on memory consolidation?
First off, a citation on the neurochemical effects of exercise beneficial to memory consolidation. Certain activities (post-study) that modulate neurotransmitter systems have generally been shown to affect how well the brain stores memories, such as caffeine consumption. Regarding the topic of your question, i.e. the effect of mental exercise on memory consolidation, it seems that post-study mental exertion has a negative effect on consolidation. This work cites a number of studies which generally show something along the lines of: "wakeful rest provided a more favorable condition for memory consolidation than the non-verbal cognitive task." Generally speaking, memory consolidation seems to work best under the influence of specific neurotransmitter release (induced by physical activity, for instance) or rest. After all, one main theory of why we sleep is that it is for the purpose of storing memories.