I refer to A.A. Hodge’s Outlines of Theology as much as any book in my library. It is a superb one volume collection of the Biblical knowledge of the 19th century titans at old Princeton. It’s more practical and manageable than, say, a three-volume Systematic Theology. Condensed and brief, it encompasses a colossal amount of Biblical knowledge.
And perhaps consider getting someone in your family a Reformation Study Bible. I wonder how many don’t read their Bibles because no one has bought one for them and encouraged them to read it?