This is a lot or 10 books by Wendelin Van Drasnen.  All are from the Sammy Keyes series.

1. Sammy Keyes and the Hotel Thief

2. Sammy Keyes and the Skeleton Man

3. Sammy Keyes and the runaway Elf

4. Sammy Keyes and the Psycho Kitty Queen

5.  Sammy Keyes and the Wild things

6. Sammy Keyes and the Holleywood Mummy

7. Sammy Keyes and the Art of Deception

8. Sammy Keyes and the Sisters of Mercy

9. Sammy Keyes and the Curse of Moustache Mary

10. Sammy Keyes and the Killer Cruise.

All books are in very good condition but may show some wear and have a name and a school sticker in the inside cover.  All have the "s" and "k" on the side as shown in the pictures.