If you are a registered user you can see what pages you have contributed to on your profile page. This page is currently very basic but does show pages you transcribed or reviewed, sorted by campaign and starting with your most recent contributions. We’re hoping to improve it soon, so please let me know any changes you would like to see!
As you've noticed, we use a collaborative model of transcription, so others may work on a page once you leave it or once 10 minutes of inactivity have passed. Saving an item means that you have saved the changes made the transcript, not that you have reserved it. You can always return to that page, but you may find the transcription further along or even completed! If someone else is working on the page, you can choose to view it, but if you make any changes your work will not be saved.
I hope this helps!