The Road to Nava

Getting into Nava
When the player leaves Lyndow and walks northeast to Nava, they arrive at a bridge, there will be a guard named Efe. Go through her speech prompts with her until she tells you that you must have three letters praising your character in order to enter the city (this quest is called Admission into Nava).

Here is a quick way to get in:

For the quest 'A Rare Bird', enter Anika's House (between Onur's house and the inn called Kestrel's Aerie) and go through all speech prompts. Then leave her house and walk toward The Great Shade (you might as well capture the painting for the main quest, too). There doesn't appear to be a way into the treehouse on The Great Shade, at least not this early in the game. You will notice a few ropes extending from this tree. The end of one of them attaches to a post nearby, and if you follow it, you will find an owl perching before the post on a log. Capture a painting of the owl (it will then fly away) and bring the painting to Anika. She will pay you 30 glowstones for the painting and generally be overjoyed. Once you approach Nava, you can return to Anika and ask her for a letter.

For the quest 'A Wounded Waterfox', talk to the bear Park Ranger Viktar (he's holding a shovel and pretty stressed out). Go through his speech prompts until he tells you about the waterfoxes and their cave (not more than a hundred paces from him). There is no direct trail there from the main path but walking to the right of him, you will find a dirt path quickly and that will bring you to the cave. If you already have found eggs, you'll be able to finish the quest. If not, keep exploring the woods until you do, then return to the cave. You will see the injured Waterfox to the left, which you cannot interact with, and the trap is to the right. You can interact with the trap, leaving an egg inside for bait. Once you've baited the trap, back up and wait for the waterfox to enter the trap. Viktar will come to the cave and you can go through several speech prompts with him and after this, he will not have more speech prompts for you until after approaching Nava for the first time. Once you approach Nava, you can return to Viktar and ask him for a letter. Also after completing this quest, in the stream and close to the Great Shade, you will see more waterfoxes in and out of the water, but you still cannot interact with them.

Enter Kestrel's Aerie for a few supplies, a place to stay for the night, and to complete a quest to get a map. The first room upstairs contains the Cartographer. Go through the speech prompts until you get the quest 'Cartographer's Ink'. All you have to do for this quest is find four Inky Cap mushrooms. If you already collect pretty much everything you see, you may have enough or close to enough mushrooms already. This is the only type of mushroom in this area. Upon returning to the Cartographer and giving her the mushrooms, she will give you a map that keeps track of small locations you have discovered and gives you an idea of where to go for later quests. If you purchase a certain kind of tea, you can also use this map to fast travel. The map will not give you a marker of your location or which direction you are facing, so it is probably most useful with the tea. If you still need letters, you can ask the Cartographer for one as well.

The only other person I've found that can give you a letter is Onur, which you can only get after talking to Bojan, completing his speech prompts, and becoming his friend. Feel free to add more if anyone finds another way to get letters!