I have both books. I bought the Mann book shortly after is was available in the U.S. earlier this year. I find the Man book to be a very useful volume because of the inclusion of the Irish Shrimp style, grub style, and Ally's Shrimps flies. These fly styles are not well known here in the U.S. or Canada; however, they are very effective fly styles.
The Ally's Shrimps especially for both summer and winter work. Granted they are not the easiest flies to tie, but they are a lot easier than a full-dressed fly or a G'P. And they are very effective and can be tied from as large as you wish to as small as is practical for steelhead or atlantic salmon.
The Irish Shrimp style is very effective during summer/fall and late spring. They can be tied in rather small sizes and have the three-dimensional movement and sillouette of all good fishing flies. They have what Alec Jackson calls the illussion of bulk when in the water.
Yes, the Mann book does not have actual photographs of the flies, and it does not have tying directions or fishing instructions for them either; however, it does provide the history of the styles and provides the pattern recipes as well. For a person who has learned how to tie body veilings, tail hackles, tented jugle cock wings, spey hackles, braonze mallard, etc. it is a superb book.
The Shewey book has the how to tie sequence instructions, and the how to fish instructions along with actual photographs of the flies to go with the fly recipes. It also has an emphasis on G.P., Spey, and Dee style flies. Shewey completely ignores the Ally's Shrimp and the Irish Shrimp style, a shortcoming in my view. However, it is a very fine book.
To these two fine volumes, I would recommend folks get Knos's "Autumn on the Spey". It is available froom Fly Fisher's Classic Library (they have a web site of the same name). It is expensive form them, about $70.00 plus airmail from England, but worth every penny because it has the old spey flies illustrated, gives the pattern recipes, and tells how to fish them as well. A most useful little tome. First published in 1872 and just as up to date and useful today.