Because everything I know about the Oregon Coast Trail (almost*) is in my new book Hiking the Oregon Coast Trail, which is now out from Mountaineers Books!
You can buy/order it anywhere. I talked the publisher into making it small format so you can use it to plan but can carry it with you on the trail too and not feel too bad about the extra weight. There is no other up-to-date resource like it for backpacking this somewhat complicated but super fun trail, whether you're doing a section or the whole thing.
*RE "almost": Literally the week it was headed to the printer I found out the northern trailhead was closing until maybe 2023 due to construction on the jetty. So I quickly edited that section accordingly. Then I went to the trailhead and checked in person, and in fact you can still start there! In other words, guidebooks are all instantly out of date, because the world is always changing and new trails are being built and old trails are falling into the ocean and ...
However I do have a website, HikingtheOCT.com, where I have posted some basic info about the trail AND where I am posting updates as I get them: new places to camp, boat ferry availability, trail closures and new trail sections, etc. Please use it to supplement Hiking the Oregon Coast Trail. My 2015 book, Day Hiking: Oregon Coast, also has tips for thru-hikers and has been helpful to many, but it's very brief and outdated in places (and frankly wrong in a few spots), so please don't use it anymore for that purpose; use this new book instead. If you're just wanting to day hike along the coast, it's perfect.