<Tr> <Th> St Martin's Theatre </Th> <Td> West Street </Td> <Td> 550 </Td> <Td> Stephen Waley - Cohen </Td> <Td> The Mousetrap </Td> <Td> Play </Td> <Td> 1974 - 03 - 26 26 March 1974 </Td> <Td> Open - ended </Td> </Tr> <Tr> <Th> Theatre Royal, Drury Lane </Th> <Td> Catherine Street </Td> <Td> 2196 </Td> <Td> LW Theatres </Td> <Td> 42nd Street </Td> <Td> Musical </Td> <Td> 2017 - 04 - 04 4 April 2017 </Td> <Td> 2019 - 01 - 05 5 January 2019 </Td> </Tr> <Tr> <Th> Theatre Royal Haymarket </Th> <Td> Haymarket </Td> <Td> 888 </Td> <Td> Crown Estate </Td> <Td> Heathers: The Musical </Td> <Td> Musical </Td> <Td> 2018 - 09 - 11 11 September 2018 </Td> <Td> 2018 - 11 - 24 24 November 2018 </Td> </Tr> <Tr> <Th> Trafalgar Studios </Th> <Td> Whitehall </Td> <Td> 400 </Td> <Td> Trafalgar Entertainment Group </Td> <Td> Misty </Td> <Td> Play </Td> <Td> 2018 - 09 - 14 14 September 2018 </Td> <Td> 2018 - 11 - 17 17 November 2018 </Td> </Tr>

How many musicals currently in the west end were originally books