Lot of 4 blue hardcover Hardy Boys detective stories by Franklin W. Dixon, nicely reprinted by Grosset & Dunlap in 1991 and 1999 with the original illustrations and typeface, to "Celebrate 60 Years with the World's Greatest Super Sleuths".  Included are:

#2  The House on the Cliff  ISBN 0-448-08902-5.

#10  What Happened At Midnight  ISBN 0-448-08910-6.

#25  The Secret Panel  ISBN 0-448-08925-4.

#42  The Viking Symbol Mystery  ISBN 0-448-08942-4