Honestly, and I'm sure you can imagine me holding my nose as I write this, the best solution for everyone's comfort is probably three books. One with only dragons, one with only shrens, one with both. (Otherwise I can only imagine the majority of dragon readers will set the book on fire the first time they notice it has a shren in it, which, while it is a hilarious mental image, is also going to make your project totally useless to them.) But if you do all three, then people can choose which one to consult according to their comfort levels.

I'm trying to think of alternate solutions involving writing it up with various kinds of information concealed, but nothing workable is coming to me and all the approaches I can think of just make it less convenient for the people without assorted hangups.