Books 1-3 in one.  Series thru 21, missing 17.  4 between the numbers books.

Three Plums in One

Four to Score

High Five

Hot Six

Seven Up

Hard Eight

To the Nines

Ten Big Ones

Eleven on Top

Twelve Sharp

Lean Mean Thirteen

Fearless Fourteen

Finger Lickin' Fifteen

Sizzling Sixteen

Explosive Eighteen

Notorious Nineteen

Takedown Twenty

Top Secret Twenty One

Plum Spooky

Plum Lucky

Plum Lovin'

Visions of Sugar Plums