I can’t seem to stop thinking of this topic and I can’t sleep. so here’s a few more recs;

• John Langan, the Fisherman. More river based horror than forest I suppose, but should fit the bill. Lovecraftian.
• Nick Cutter, The Troop. have not read this one myself, though I’ve seen it recommended in horror lit circles often enough. Has a reputation for including scenes of graphic and shocking violence.
• Stephen King, the Girl who Loved Tom Gordon. Have not read, but have heard great things.
• Ramsey Campbell, the Darkest Part of the Woods, another I haven’t read, but I’ve heard great things.
• Karl Edward Wagner, Sticks. Short story. I’ve only read a few stories by this author. Overall I wasn’t very impressed, but this one was a stand out. Rumored to be an inspiration for the Blair Witch Project, but if so I didn’t pick up on it at the time.
• Arthur Machen, the White People. Short story. Probably a stretch to include, but it came to mind. Occult/pagan horror that takes place, in part, in the nearby woods.

Are you set on forest horror, specifically? If we expand it to wilderness horror, I can think of a few more:

• Jeff Vandermeer, annihilation. Cosmic weirdness in the Everglades (possibly? The location was never specified that I can recall). This one seems to be love it or hate it. I personally loved it. There are sequels, but they are inessential and frankly superfluous.
• Scott smith, the Ruins. Jungle horror. A group of tourists are trapped on Mayan ruins surrounded by carnivorous plants. Haven’t read the book but I enjoy the movie.

Are you willing to expand the concept of wilderness horror even further to ice/snow horror?

• Dan Simmons, the Terror. Inspired by the real story of the HMS Terror. Trapped in the ice while in search of the fabled northwest passage. Facing starvation, mutiny, and lead poisoning, and also, there’s something out there stalking them on the ice. Have not read this, but I highly recommend the amc miniseries adaptation (the first season - there are follow up seasons that are unrelated but carry the same title. Don’t ask me why).
• HP Lovecraft, At the Mountains of Madness. Ive read this long enough ago now that I’m fuzzy on the details, but I think it’s safe to say it is full of unnameable horrors, gibbering madness, non Euclidean geometries, turgid prose, and appalling racism.
• John W Campbell, Who Goes There?. The basis for John Carpenter’s The Thing. (And earlier, Howard Hawks’s The Thing from Another World).

Enjoy your camping trip!