Ok so I started reading the Stephanie Plum books after I read Wicked Appetite because I wanted to find the back story for Diesel.  I was curious how many of the mysteries that are Diesel had gotten solved already in the Plum books.  After reading the first four I'm a little sad he's not been in any yet.  Here is the reading order I've found firstly I'd like to know if its correct or the best way to read the books:

One for the Money
Two for the Dough
Three to Get Deadly
Four to Score
High Five
Hot Six
Seven Up
Hard Eight
Visions of Sugar Plums (between-the-numbers holiday novella)
To the Nines
Ten Big Ones
Eleven on Top
Twelve Sharp
Plum Lovin’ (between-the-numbers holiday novella)
Lean Mean Thirteen
Plum Lucky (between-the-numbers holiday novella)
Fearless Fourteen
Plum Spooky (between-the-numbers novel)
Finger Lickin’ Fifteen
Sizzling Sixteen
Smokin’ Seventeen
Explosive Eighteen

Secondly does Diesel first show up in Visions of Sugar Plum's the first in between novels?  Thirdly is he only in those in between novels or does he make an appearance in the number novels too?  And Lastly I'm enjoying the regular novels but I'd like to read the in between novels before I read the next Diesel novel so if I just read the in between novels first does it give too many spoilers cause I'd rather not do spoilers if I could avoid them?    

Diessel is only in the in between novels. I don't like the in betweens so can't tell you if he is in all of them or if not reading the regular series makes a difference in the flow.

The Stephanie books are now one through nineteen.  As Dorothy said he's only in the between books and I'm not sure if he's in all of them.  I don't think so though.  I think, a really vague memory, he might be in one, maybe two of the numbered Plum books but has a very small part.  To the point where I wondered why in the world he was even mentioned.