By Kage Baker. This is the second book in her Company series.
I almost didn't finish this book because of the silliness and disrespect in it, but I picked it up again last night. I haven't read any of the other books in the series but took the liberty of looking them up on Amazon.
The premise: The Company known as Dr. Zeus has discovered, in the 24th century, a way to travel back in time. The only limitation is that RECORDED history cannot or must not be changed (it isn't entirely clear which is the truth). They've also discovered a way to make humans immortal, but since the process is expensive and involves radical surgery which has to be started in infancy, there isn't really anyone who's interested in it. So, they send a team back in time to recruit some people from the Stone Age, turn them into immortals, and leave them to travel through time the normal way (i.e., by aging) to loot the past for the benefit of the company.
The actual plot of this book is not very interesting. The interest lies in the hints that are dropped about the Company's real purpose, which are fairly few and far between. Even though Mendoza isn't the main character, she seems to be one of the really important ones, given how much Joseph cares about her and also by the fact that she seems to be involved in a major part of the company's true purpose (thanks Amazon for information about later books) all unknowing.
This one seems to be a toss-up: I'm not entirely sure it isn't worth reading, but I can't recommend it wholeheartedly either. It definitely isn't worth reading unless you also intend to read the rest of the series (or maybe look for spoilers on Amazon).