There is some dispute over the correct order in which to read these, as the publishing order is quite different from story order. The only area of universal agreement is that Dead Witch Walking comes first, followed by The Good, The Bad, and The Undead. After that, things get complicated.


Here's the order from wikipedia:
Reading order by publication date
     1.  Dead Witch Walking
     2.  The Good, The Bad, and The Undead
        2.1  Undead in the Garden of Good and Evil (Ivy novella in the Dates From Hell anthology, also in Into the Woods compilation)
     3.  Every Which Way But Dead
     4.  A Fistful of Charms
        4.1  Dirty Magic (Mia short story in the Hotter Than Hell anthology, also in Into the Woods compilation)
     5.  For A Few Demons More
        5.1  The Bridges of Eden Park (Kisten/Rachel short story in For a Few Demons More(US mass market edition), also in Into the Woods compilation)
        5.2  Two Ghosts For Sister Rachel (Robbie/Rachel/Pierce novella in the Holidays Are Hell anthology, also in Into the Woods compilation)
     6.  The Outlaw Demon Wails (retitled Where Demons Dare in the UK)
        6.1  The Bespelled (Ceri/Al short story in The Outlaw Demon Wails (US mass market edition), also in the Demons anthology and the Into the Woods compilation)
     7.  White Witch, Black Curse
        7.1  Ley Line Drifter (Bis/Jenks novella in Unbound anthology, also in Into the Woods compilation)
     8.  Black Magic Sanction
     9.  Pale Demon
        9.1  Blood Work (Graphic novel of Rachel's internship under Ivy at the Inderlander Security)
        9.2  The Hollows Insider (The Hollows world book - an overarching new story, character profiles, maps, spell guides, charm recipes, secret correspondence, ... )
     10. A Perfect Blood
        10.1 Million-Dollar Baby (Trent novella an Into the Woods original)
        10.2 Blood Crime (Graphic Novel of Rachel's continued interning under Ivy at the IS)
        10.3 Trouble on Reserve (pre-installed on Sony eReader for iPhone, iPad and iPod Touch)
     11. Ever After
     12. The Undead Pool
     13. The Witch With No Name 


Chronological reading order
     0.  Novellas happening before first book
         0.1  The Bespelled (How Algaliarept took Ceri to the ever-after)
         0.2  Two Ghosts For Sister Rachel (When and how Rachel and Pierce meet)
         0.3  Undead in the Garden of Good and Evil (Ivy's rebuff of Art and when she and Rachel begin to work with each other)
         0.4  Blood Work (Graphic Novel of Rachel's interning under Ivy at the Inderlander Security)
         0.5  Blood Crime(Graphic Novel of Rachel's continued interning under Ivy at the Inderlander Security)
     1.  Dead Witch Walking
     2.  The Good, The Bad, and The Undead
     3.  Every Which Way But Dead
     4.  A Fistful of Charms
         4.1  The Bridges of Eden Park (Rachel helps Kisten to save a child of his sister)
     5.  For A Few Demons More
     6.  The Outlaw Demon Wails
         6.1  Dirty Magic
     7.  White Witch, Black Curse
         7.1  Leyline Drifter (Jenks and Biz's adventure with a dryad, a nymph and a pixie family in need)
     8.  Black Magic Sanction
     9.  Pale Demon
         9.1  Million-Dollar Baby (How Jenks and Trent rescued/kidnapped Trent's daughter)
     10.  A Perfect Blood
     11.  Ever After
         11.1  Trouble on Reserve
     12.  The Undead Pool
     13.  The Witch with No Name