Add function to read book IDs from stdin

refine.go

 package liber
 
 import (
+	"bufio"
+	"bytes"
 	"fmt"
 	"io"
+	"log"
 )
 
 func (db *Database) Refine(dir string) error {
@@ -29,3 +32,22 @@ func (db *Database) ListBooks(w io.Writer, matchFuncs ...func(book *Book) bool)
 		return nil
 	})
 }
+
+// WithBookIDs calls a function on books whose IDs are read from the
+// specified io.Reader.
+func (db *Database) WithBookIDs(r io.Reader, f func(book *Book) error) error {
+	scanner := bufio.NewScanner(r)
+	for scanner.Scan() {
+		id := ParseID(string(bytes.TrimSpace(scanner.Bytes())))
+		book, err := db.GetBook(id)
+		if err != nil {
+			log.Printf("error: no such book %s", id)
+			continue
+		}
+		if err := f(book); err != nil {
+			log.Printf("error: %s: %v", id, err)
+			// Stop?
+		}
+	}
+	return scanner.Err()
+}