Stardew Valley: Secret Woods
Where to Get Hardwood and Books from Worms
The Secret Woods is a great place to visit regularly in Stardew Valley. You'll find ample hardwood, plenty of areas to dig up with the pickaxe (and thus many opportunities to find books). It is also home to a mysterious statue that gives one of the game's Stardrops, which increase your character's maximum energy by 34.
How to Get Into Secret Woods
To unlock the Secret Woods, you must first upgrade your Axe to Steel or higher. You'll find information on acquiring metals for the upgrade on the linked tools page. Once this is done, you can smash the log blocking the entrance. Bring a weapon, your hoe, and with your axe for this trip. Head through the southern exit of your farm. You'll be in Cindersap Forest. Now head directly to the left all the way to the edge of the map. You can use the axe to smash through and enter the Secret Woods, previously denoted by ???? on the map.
Hardwood and Worms
The woods contain 6 stumps that regrow each day, giving a total of 12 hardwood. As far as I know you do get foraging experience for these (actually a decent amount), but also from bringing the hoe and digging up the worms here. This place almost has at least 3 worms, so you can round out the museum's book collection and learn more about the game. While the first four stumps are obvious, you get to the last two by walking through the hidden passage shown above.
The area always has at least a few slimes, so you can get some combat XP. They're not really dangerous so long as you're armed with a proper weapon.
Fishing Spot - Catching Woodskip
The pool in the Secret Woods is home to the Woodskip, one of the rarer types of fish in Stardew Valley. It's used for at least one bundle at the Community Center.
The Statue, Stardrop & Sweet Gem Berry
Far on the left side of the Secret Woods, you can investigate a statue of Old Master Cannoli. It says, "Old Master Cannoli -- Still searching for the sweetest taste". If you bring it a Sweet Gem Berry, you'll get a Stardrop in return, raising your maximum energy permanently.
The Traveling Cart can sell you a 'Rare Seed' which will let you grow these yourself. Visit it every Friday and Sunday. The Traveling Cart appears on the way to the Secret Woods - you can't miss it. Every now and then the rare seed will be available for purchase for around 1,000G (for seeds) to 12,000G (for a fully grown plant). Sweet Gem Berries grow from Rare Seeds in Fall and take 24 days to grow, so you will want to plant it very early in the fall, by the 4th day.