To Hallgreening
Think you're referring to Mr. American - a great read as are all GMF's books - though not really part of the Flashman series although the elderly Flashy does of course appear in it.
I would highly recommend you read all the Flashman books as well as everything else written by this brilliant story-teller.
If I had to recommend just 3 I'd go for:
1. Flashman - the first of the series which takes up Flashman's story from the point he is expelled from Rugby School (ie. as described in Tom Brown's Schooldays.)
2. Flash for Freedom - dealing with Flashy's experience of the slave trade.
3. Flashman and the Redskins - this follows on exactly from Flash for Freedom so should be read after it.