Is zakat due annually on a shop and the merchandise that may remain for a year, or is only sadaqah sufficient? What is the jurisprudential rule for paying zakat on this type of trade, and what are the useful books in this field?

Zakat (definition) is not obligatory on what you have in your shop—which are trade goods—except under two conditions: that a full lunar year (hawl) passes over them, and that their value reaches the nisab (definition) (which is equivalent to 85 grams of gold). At the end of the year, the value of the goods is taken into account, not the goods themselves.