With SharePoint allowing security to be managed all the way down to the item/document level, it can easily become quite a mystery to users- just who has access to what exactly? The best way to avoid this confusion is to avoid item level security and only break permission inheritance at the libray level when really necessary. Below is a neat little way (I think) to show users who has access to the library they are currently working within, just in case they want to think twice about sharing.
Create a new document library view called Library Permissions. Open the new view and edit the view page. Minimise the document library web part and above it either add a CEWP to describe the level of access and add links to the relevant groups OR a Site User web part configured to show the Contributors group on the library. This view is a quick and easy way for users to access this security information but be aware that the library tabs will not function on this view page (unless the user clicks the minimised library web part). The view breacrumbs can be used to navigate back to the library. There is a small maintenance overhead as well. If extra groups are added to the library permissions then this view would need to be updated to reflect this. The best thing about linking to the Contributor group though is that the members will likely change but not the group itself.
Alternatively you can add a description and link in the list/library descritpion field (settings). This isn’t quite as pretty as links in the description field can not be entered as HTML so the link is displayed in full.
I have implemented some sites with “security folders” because of the need for fine-grained permissions. In thoses cases I have created a custom folder content type : Secure Folder and added a mandatory people/groups field. This can then at least be displayed against the folder in all views. Again, there is a maintenece overhead here but it worth it if you are breaking permissions at that level.