As an alternative to glue, if the wood is thin could you drill tiny holes in it and sew the quill of feather down? The stitching would be decorative and part of the design.
Heehee - it just feels wrong, doesn't it? I'm thinking about altering some printed books, too. I like the ones that have art done on the printed pages, holes cut into the book to make hiding places, and so on.
For plain (unprinted) paper I'd suggest slightly damping it and pressing under weights for 24 hours - not sure if this is suitable for your printed pages, however.
Great envelopes! I love making envelopes out of magazines, too (mostly Country Living & the Gudrun Sjoden clothing/homewares brochure which is printed on lovely paper).
I use them as keepsake storage, and as gift wrapping.
Never as 'external' envelopes - too worried about them being too flimsy or getting damaged, so I always use them inside another plain, bought envelope.
I love owls too. Yours look so innocent. Are they really, or did they have their beaks in the biscuit tin while your back was turned, and now they're *pretending* to be innocent?!