Grant Fritchey has a book on execution plans, revised in 2012, available as a PDF for free from the Redgate site where print and Kindle editions are also available for purchase. There are MANY free books available on Redgate’s site, highly recommended reading!
And there’s SQL Sentry PlanExplorer, in both a free and paid version, it’s an amped-up version of the basic SSMS tools. Like Mladen’s tools mentioned in the previous post, it is invoked by a right-click on an execution plan object and opens in its own program window. There’s a lot of interesting aspects to this tool, such as the Join Diagram that shows you sort of a relational diagram of your query. One thing that’s quite nice is that each operator’s cost is shown above the icon with the most expensive operator in red and the next in yellow. If you hover your mouse pointer over any tool you’ll get the same information as what you’d see in SSMS, but if you hover over the first (top left) icon, it will tell you if a good enough plan was found or if the optimizer timed out.