Whilst it is best to read second foundation after 'foundation' and 'foundation and empire' it is not essential, as the prologue fills you in on the basics of the previous two novels.
The book goes a long way to explain the purpose of the mysterious second foundation, but the main focus is on its location, and the search for it by both the mule and the first foundation.
The first half puts order into the chaotic situation left by the events in 'foundation and empire', and follows many twists and schemes that make it entirely engrossing, without giving the slightest hint of the dramatic ending.
The second half provides the real substance of the book, a rebellion of the first foundation against the ideals of the second foundation. This leads the reader to question the entire idea of psychohistory, as the first foundation does not like to think of itself as being 'controlled' by the second, even though it knows that the interference of the second foundation will result in a bright, peaceful future for mankind.
The book is laid out with a new twist to the plot in every chapter, making it almost impossible to put down, especially towards the end.
The most interesting parts are meetings of the second foundation members, which explain the inner workings of the seldon plan, along with how the arrival of the mule affected it.
The characters are well presented, and are developed only as much as is necessary, to keep the fast pace of the novel going, but are entirely believable, and link in with the characters in the previous books.
Some people like to read the last page of a book first, doing this with second foundation, and indeed with the entire foundation series, will ruin the book utterly.