There is a concise run through the perils of Methaemoglobinaemia by Andy O’Neill, who runs the Emergency Medicine Ireland blog. If you want more detail, eMedicine.com and UpToDate.com also provide high yield information.
It’s not a particularly common presentation; I saw a couple and a young soldier present within 3 years of each other and haven’t seen another one since. But things that are clinically rare are not always rare in exams and Andy’s review will have you up to speed in under 4 minutes.