GB: Leathers when over dried at a too hot of a temp will cause them to become brittle. The book is just a guideline it's not the bible. Humidity, different food purees, ingredients all need to be taken into account. Store bought leathers are full of sugars. That's where the shine comes in.
Give it another try. Lower the temp considerably. Dehydrate one select fruit at a time. Some have more water content then others. They will not dry at the same time if done as a mixed group. Start the temp at 100 degrees. The puree should be 1/8 to 1/4 inch thickness. Dehydrate low and slow, that's the key.
Over ripe foods worked better for me then the just picked. It does take some experimenting to achieve the ultimate goal.
Here's a few links that might help. I completely forgot about concentrated juices!
Yes you can cook the puree down.
Making Fruit Leather or Fruit Roll Ups with a Food Dehydrator - Food-Dehydrator.com
How to Make Fruit Leather
I'm really liking some of those recipes. Wow.. :)