Reddit mentions of Fishing Yellowstone National Park: An Angler's Complete Guide To More Than 100 Streams, Rivers, And Lakes (Regional Fishing Series)

I don't have any recommenation on guides, but I was in Yellowstone in August of 2017. I had never fly fished before and had 1.5 months before the trip to teach myself.

Some pointers in case you want to venture out on your own and fish.

• Any spot 5-10+ minutes walk away from parking or roads = drastic reduction in crowd. You'll be there during one of the busiest months.
  
  
• This book was super helpful: https://www.amazon.com/gp/product/1599211424/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
  
  Figure out where you will be in the park, buy the appropriate flies, profit.
  
  
• Practice cast for sure. You don't need to learn any crazy teqniques. Just be able to make an acurate 30-40 foot cast. No hauling. Check out some Orvis casting lesson videos on YouTube.
  
  
• GO TO SLOUGH CREEK! I don't know if your an avid/hardcore angler or outdoorsman, but even if you just appreciate the beauty of nature away from roads, crowds, the modern world, etc, this place is magical.
  
  A few pictures from my day there:
  https://imgur.com/a/3Xte1?
  
  Happy to share more details if you are interested.

I was just there last week and had some decent fishing. While I can't speak to the patterns specifically, I can say from my limited experience, try to avoid crowds as much as possible. That stretch of river through the meadow right next to the road. Looks beautiful, right? Well every other idiot with a fly rod all summer thought the same thing and fished the living daylights out of that section. I found that even walking 1/2 mile away from the road the fishing was better than the obvious spots. If I had the time I would do some serious back country hiking to some underfished stretches of the Yellowstone itself. Or hike up the Lamar upstream from the confluence of Soda Butte creek. If you haven't bought the book, buy the book and be sure to stop in a [Park's Fly Shop] (http://www.parksflyshop.com/index.html) in Gardiner, MT mere yards from the North Entrance. These guys literally wrote said book, and put me on fish both times I went in for advice last week. Good luck.