Usually I have many pictures for a serendipity post but this find was so cool I thought it was enough on its own. It's a book from 1903 called Artistic Homes. It has images of homes with their blueprints right under them. It seems like it was made for player's handouts for your mystery, Cthulhu, or time travel games:
The book has a testimonial section in the back with a bunch of letter facsimiles you might be able to use as well. Granted, they are all saying pretty much the same thing "blah blah blah the house you built us is wonderful" which is hardly a clue to a nefarious plot, but for some of them they have interesting letterhead and signatures you could use if you replace the text with something of your own. Like this one:
These images are all in the public domain and you can use them however you wish. I hope you find them useful.