The collection is organized thematically, so there's a chapter with architectural details, one on drawings from Rome, one on drawings of people, one with Swedish land- and townscapes, etc.
Visiting Rome to look at antiques and ruins was almost de rigueur for an architect/artist in those days, but your drawings from the place didn't have to be done on the spot; you could just as well copy somebody else's drawings from the place. I guess the important thing was to document the various form elements so you could use them in your own work later on. Ruins were also considered rather romantic. This is a church, S. Maria del Sole:
Still, even that which isn't Rehn doing original work displays his magnificent drawing skill. This is a memorial over a Cardinal by the French architect Bouchardon:
Most interesting to me is still the chapter with Swedish milieus, and it's a real shame that these pictures aren't available on the web in larger format, as they are very interesting depictions of what various parts of our country looked like in the 1700s, beautifully executed. This one is from my home town of Uppsala, with the so-called "peasants' church" in the foreground:
If all the pictures had been of larger size, I would have recommended this to just about everybody. As it is, I guess it's only if you have a special interest in Rehn, 1700s interior decorating, or an extraordinarily skilled draughtsman of olden times that this is a book for you.
But man, that guy could draw!