Murder/mystery woman escapes from library

I'm trying to remember a book I read 5-10 years ago. I think the genre was murder/mystery. My memory about the storyline is a little hazy. The story was about a kept woman who gave the perception of being naive and helpless. Her boyfriend was some kind of criminal. He had an associate, who was some kind of undercover cop/FBI. The boyfriend gives her to the associate for a few hours as compensation. She pretends nothing happened and convinces the boyfriend she is heart broken that he would treat her so. I also remember she goes to the library, disguises herself in order to get away. Does this book sound familiar to anyone?

Book Found: Death Angel by Linda Howard

that sounds like Death Angel by Linda Howard
she's a criminal guy's mistress, he gives her to his assassin one afternoon as a reward, nookie with assassin happens and he leaves without her, she escapes with all of bad guy's money (she uses a library to facilitate her escape) bad guy engages assassin to take her out.

That is the book. Thank you.

>1 Niobi: I think editing the original post to announce the book has been found is an excellent idea.