Literally, solicitation means: 'urgently asking'. It is the action or instance of soliciting;petition, proposal. In criminal law, it most commonly refers to either the act of offering goods or services, or the act of attempting to purchase goods or services,
basically if you are trying to buy/sell for personal gain weather its monetary, economical or social status
so saying "hey this is a good book u should read it" does not fall under solicitation unless you own the rights to the book and would therefore gain money through the purchase of the book (via your suggestion)