What is the best way to secure the last course of asphalt shingles to a simple lean-to shed roof with no ridge or flashing because it is a stand-alone wood shed? Roofing tar/adhesive alone? Or tar/adhesive over roofing nails which secure that last course?

I have a roof on a shed that is leaking and was wondering how this should be fixed properly. Here are links to two pictures: [url]http://www.box.net/shared/77zz7yvks8[/url] [url]http://www.box.net/shared/qxuo4stgk0[/url] I think the roof may have pulled away from the block wall it is attached to. I guess it would need to have counter flashing set into the block wall. I think the clay tiles on top of the wall are also leaking allowing water to seep into the block wall. I don't know if you can tell from the pictures, but there is a cap block that sits out a few inches over the regular cement blocks. On top of that sits the clay tiles. What is the best way to fix that? Any advice would be really helpful.