First of all, these books definitely do not work as stand-alones. It's been awhile since I read the last book in this series and I was really floundering as I struggled to remember characters and previously revealed plot twists that are all part of the series arc plot line.
The story is pure adventure from first page to just about the last and the number of characters involved, the different sides playing off each other, and all the bad-guy-or-good-guy stuff, along with writing that can sometimes be a tad confusing, makes this a book you have to be invested in to enjoy.
But it is fun. Sometimes it's over-the-top, and the battle of the sexes angle is tiresome, although thankfully not overdone, but mostly it's just a fun action/adventure mystery revolving around art and forgeries and thieves and international intrigue. A great type of book when you're looking for a quick light read.