Converta-o para LEFT JOIN e coloque a(s) condição(ões) adicional(is) lá
SELECT DISTINCT d.Name, t.Buyer, t.Seller FROM Book AS b
LEFT JOIN TradingDesk AS d ON b.TradingDeskId = d.Id
LEFT Join Trade as t
on substring(t.Seller,2,len(t.Seller)) = cast(b.Id as varchar)
AND t.Seller like 'B%' -- put your additional condition(s) here
UNION
SELECT DISTINCT d.Name , t.Buyer, t.Seller FROM Book AS b
LEFT JOIN TradingDesk AS d ON b.TradingDeskId = d.Id
LEFT Join Trade as t
on substring(t.Buyer,2,len(t.Buyer)) = cast(b.Id as varchar)
