Hi Michelle,
I think that although the books are best read in order, any one of the books can probably be picked up by itself and still be enjoyable. Jane Rizzoli, the primary character, has changed quite a bit through the series, though, and if you want to see the transitions she's made, it's best to start with THE SURGEON, where she's first introduced. In the first book, she will probably not seem very likable. In truth, she's .. well, a BITCH is the best word for her. But as the series continues, and she finds true love and becomes pregnant, and then a mom, you see her grow happier and become more fulfilled and become far more likable.
I didn't intend for this to be a series. In truth, Jane was supposed to die in THE SURGEON, and I had her death scene all plotted out. But then when I reached the climactic scene where she was supposed to die, she did something unexpected. She fought back... and she survived. She became a real person to me, and refused to surrrender, and that's the characteristic that's defined her for me. She's a survivor, and a fighter, and she deserved to live. And so she did.
Which is how the series was born.