I recently started reading LiteMind, and have been enjoying it very much. The focus of the site is personal development, productivity, freedom, and creativity ... all topics that product managers can benefit from.
They have an offer today that you should check out: A free eBook just for making a comment on a post. I think it's a draw, so it's not a sure thing, but the list of books is worth perusing.
Check it out here: http://litemind.com/personal-development-giveaway/