I'm not a carpenter, barely an 'about the house' handyman, but I have 2 little kids, and a space to put a playground or swingset by Walmart or better- build a treehouse. I've always wanted a treehouse. By chance on Google, I discovered the plans for Stiles' A-frame treehouse graciously offered free online. And with little know how, but committed, I followed the brief instructions, working on the weekends, to the tee, and by golly, we have a beautiful treehouse.
This book I bought later, and it contains the original A-frame treehouse among other designs and indeed 'other Cool Stuff', e.g., accessories for the treehouse, a working cannon for kids, a simple boat, unusual swings, homemade toys etc. There are some color photos of the creations, cute drawings, and the simple schematics. There's info on basic construction and woodworking and they give you just enough to put it together. I learned as I went. I own their other book 'Treehouses you can actually build'(never used it-intended to- until now), another Stiles' gem, and found it very helpful.
A few suggestions from a total novice:
1.Right tool for the right job. Few tools are needed. They list them for you, e.g., circular saw, jigsaw, 2 electric drills, level, and so forth. Buy good equipment, good blades-lots of teeth, screws- I like the ones with star head inserts, less slippage, etc.- it helps immensely.
2. Get help- to hold poles, lift frames, give insight. I built the A-frame treehouse alone, Lone Ranger style- doable but somewhat hard. Stay focused; keep attention to detail, review every aspect of the drawings. Not all the minute specifics are given. You must think out your steps, review different types of screws and materials and how to approach the next task especially if you don't know anything like me. The Home Depot/Lowes' crews helped in that department plus Googling.
3. "Plumb and Level"- get the foundation and the frame right and the rest is much easier;
4.Plan for safety- soft ground cover like chopped, colored rubber; a higher railing for the treehouse patio; extra wooden bars for the windows if kids falling out is an issue;
5. Modify the plans if reasonable- my wife suggested the staircase rise up into the treehouse not alongside it with a 'secret' hatch(the Stiles recommend similar in their other book); I used redwood instead of cedar railings, added another small window in the back with a bell to ring...
6.Take your time and have fun or do something else!