As luck may have it, I have been seeing people at Barnes and Noble and Costco who looked confused and had the same question.
Really quickly....of the authors I have read (see the list to the right) here are those that do need to be read in order. When I get time, I'll list the books in order, but, for now you can probably get the order at the authors web site.
1. My all time favorites: Perri O'Shaughnessy's series about Nina Reilly. The first book in this series is "Motion to Supress". 5 WaterTowers....I love this series and I love Nina and the two Perri's.
2. Sue Grafton's Kinsey Millhone series starts with "A". That one is easy! Start with "A is for Alibi" and move on from there. 3.5 to 4 WaterTowers.
3. D.J MacHale's Pendragon series. Another 5 WaterTower series. Start with "Pendragon The Merchant of Death".
4. J. K. Rowling's Harry Potter series (but I bet everyone knows that). 5 WaterTower series, start with "Harry Potter and the Socerer's Stone".
5. Vince Flynn's Mitt Rapp series. 4 or 5 WaterTowers. Mitch Rapp rocks. Start with "Term Limits".
6. Barry Eisler's John Rain series. New to my list of favorites. 3.5 to 4 WaterTowers (I'm still getting used to John Rain). Start with "Rain Fall".
7. Brad Thor's Scot Harvath series.
8. C. J. Box's Joe Pickett series.
9. Mike Lawson's Joe DeMarco series.
10. Steve Martini's Paul Madriani series.
There are a number of authors who are predominantly "stand alone" but who have written a few books in a series....
11. Stephen Frey's Christian Gillette series ran for three or four books.
12. David Baldacci's Oliver Stone (Camel Club folks) is one series...another is Sean King and Michelle Maxwell...both of which need to be read in order.
I'll add to this as I have time....
Best of Reading!