I have been looking (several years ) for embellishments for a theme - mystery. I have asked many, many times at archiver's - no luck. I did a Trixie Belden book of my granddaughter posing a Trixie and can find no embellishments to complement it.
— Winnie, MI on Feb 20, 2012

Unfortunately, we do not have any mystery-specific items. However, we do have a couple suggestions for you. The first is to choose embellishments that fit the time period. October Afternoon has some really great "vintage" items that might fit your project:

http://www.archiversannex.com/Sidewalks-Miscellany-Embellishment-Assortment--DC148/default.aspx?PageID=21&CategoryID=128&ProductID=44596&SPG=2&ItemID=89440&RootCatCode=11550

http://www.archiversannex.com/Farmhouse-Paper-Pad-8X8-48-Sheets-2-Each-Of-24-Designs-PP724/default.aspx?PageID=21&CategoryID=50&ProductID=49335&ItemID=95530&RootCatCode=11150

We also have a bunch of October Afternoon papers and embellishments available in stores.

If you haven't checked it out already, we suggest taking a look at the information page on the Trixie Belden website (www.trixie-belden.com): http://www.trixie-belden.com/resources/information.htm
You might be able to create some of your own embellishments using different things from the website, such as images of characters or book covers.

You could also use items such as question marks, magnifying glasses, maps, flashlights/candles, etc to embellish the page, depending on the specific items featured in that particular Trixie Belden story.

Hope this helps!