There are 6 books in "The Sundering" series, but only "the companions" is by R. A. Salvatore, and then there is a single book called "Night of the Hunter". Do I read the whole "Sundering" series and then the last book or just "The Companions"?
They can be read in any order without spoilering earlier or later novels in the series.
According to Paul Kemp, author of "Forgotten Realms : The Resurrection", the final installation of "War of the Spider Queen" series and the "Erevis Cale" novels (as well as Webmaster of the official Forgotten Realms website and wikia) the Sundering novels are intended to stand alone with no continuous plotline, nor any substantial thematic connection between them.
During all this [e.g. the 'Era of Upheaval'], the six novels of The Sundering series tell their stories — smaller stories, stories that show the Sundering through the eyes of both ordinary folk and some of Faerûn's iconic heroes. Each novel in The Sundering is self-contained.
There are some connections between the novels — some Easter eggs, if you will — but each stands alone. What they share is a common background, with all of them occurring against the backdrop of the Sundering.
For the record, one of the books you've mentioned ("Night of the Hunter") isn't part of the Sundering series but is, in fact part of the larger Forgotten Realms series known as "the Legend of Drizzt". This book should be read at the end of "The Sundering" Series of books and forms the first book of a new short series known as the "Companions Codex" shortly to be followed by "The Rise of the King"