As Ian says Collins is about the best available, however there are two excellent guides which concentrate on confusion species they are--
1 The Macmillan Field Guide to Bird Identification - Alan Harris, Laurel Tucker and Keith Vinicombe.
For rarer migrants and for birding abroad --
2 The Macmillan Birder's Guide to European & Middle Eastern Birds - Alan Harris, Hadoram Shirihai & David Christie
Both have excellent id sections on warblers. They are to the best of my knowledge long out of print but you will find them on the internet at Amazon or ebay. Several copies ( second hand softback version) of at least the first one are currently available on Amazon with both the 2 reviewers giving it 5 stars.
Fortunately even the most experienced of birds have to place warblers flying away in the " what the 'eck whas that category" !
It's just an opinion but I'd always go for the Colling Bird Guide. Not photos nor portraying them flying away (none does) but far more importantly it's the most accurate, up-to-date and most easily useable guide out there. Few warblers are easy to identify on a flight view from the rear and no field guide is going to help you much there I'm afraid!
What would be the best book to get for the identification of british and migrant warblers? Specifically with pictures (preferably photos) of the birds flying away, as this seems to be the view I get of them more often than not!
