BLACK PLAQUES are not to be found proudly mounted on a wall - and for good reason. What with their commemoration of a brutal execution outside Westminster Abbey, the selling of sex toys in St James's Park and an intruder at Buckingham Palace with Royal undergarments stuffed down his trousers, this is not sort of historical subject matter that authorities choose to grace a building's facade or depict on a visitor information board. In fact, many might hope that such indecorous and inconvenient episodes remain quietly overlooked. But this book jogs such artful lapses of memory and at more than one hundred locations across London, Black Plaques lift the carefully placed rug to discover an unsightly, but strangely beckoning, stain.