Advice please: What plants that will compliment, can I plant alongside :
(a) Tall Campanula
(d) Day Lilly
As these are all summer flowering maybe something of interest for other seasons - how large is the flower bed? Without knowing the size/site/position etc I'd be thinking underplanting/interplanting with bulbs for spring - daffodils, tulips; maybe a shrub or two which are evergreen for winter interest or which flower in winter/spring - pieris, camellia...again without knowing your soil, whether the bed is an island or has a hedge/wall behind it .....some more detail will give you much better advice.
I'd certainly add some grasses to the mix.
Sorry I cant help with an answer to the Q but what a lovely title for a book!