An elegant collapsible book stand. It was designed with 3D-printing in mind, so there are (almost) no steep overhangs or bridges and no unnecessary filament consumption.
Make sure, that your printer is well calibrated, so that the two parts fit nicely together. Since version 1.2 the joint has a very tight fit, so it doesn't fall apart on its own. If you know your printer to over-extrude, lower your extrusion multiplier or use the "horizontal expansion" feature of your slicer with a value of about -0.02 to -0.05mm!

magnets:

Since v1.2 there is a variant of the book stand that has two hidden magnet (disc 8x3mm) slots inside, so the two parts cohere while they are stored. To install them, you have to pause your print at the layer before the slot gets closed. For a print with 0.25mm layers that is layer height 4.75mm. You can do that manually or insert a M601 g-code at the right layer.
Then put a tiny drop of superglue in the slot and then the magnet. Doublecheck the polarity! Resume the print afterwars and you're done.

Troubleshooting:

The first layers of the large rounding below the joint can be hard to print, because the chamfered edge produced an overhang that's a little too steep. I included seperate files with support enforcers for these areas, which should help immensely.

changelog:

• v1.3
  extended "foot" further back, so the book stand can hold larger books
  added support enforcers for difficult rounding/chamfer
• v1.2
  added two magnet slots (disc magnet 8x3mm) per part, so they stick together on their own while stored
  decreased clearance of the joint. The two parts now have a very tight fit!
• v1.1
  slight changes of chamfered edges
• v1.0
  initial release

NOTE: I'm trying to work out a solution for books that tend to fold on their own. Sadly I don't have a lot of time right now, so it might take a while...

Feel free to share your experiences in the comments!