It's still a huge area, can you give us a bit more of an idea what you are interested in?
http://www.seawings.co.uk/ is a great resource.
Sea, Sky & Stars by MJ Hardy on Grumman is pretty good on Grumman without the Putnam's obsession with numbers.
British Flying Boats by Peter London is a good overall history of the area of the title.
In Dutch, Dornier Do 24 by Pieter van Vijngaarden & Prudent Staal.
There's a couple of good histories on the Empire 'boats, one available on PDF form free on the web.
http://www.users.waitrose.com/~mbcass/Adventurous Empires: The Story of the Short Empire Flying-boats by Philip E. Sims
Maritime is No.10 is the official history of 10 Sqn RAAF based in the UK with Sunderlands from the start of W.W.II to the end. Ivan Southall writing on flying as a Sunderland captain is the best account of what it was like to be there. Corsairville is a fascinating read, Osa's Ark likewise.
The VP Navy website is excellent for that material, and hints to related publications.
The old MacDonald series Fighters, Bombers, Floatplanes and Flying Boats ...of W.W.II series are good on obscure French, Russian, and Italian types, for instance.
Performance and test pilot data of many W.W.II era types is covered by Eric Winkle Brown's books.
And so on...