What's the Name of That Book??? discussion

The book starts with our protagonist (teenage boy, probably named Alex) walking down a 21st century street. He notices a sign in a shop window that says, "Adventurer's Wanted," and goes inside to ask about it. The shopkeeper immediately gives him the aforementioned magic satchel and basically kicks him out a magic door that leads to another world of medieval fantasy and magic wonder. He gets recruited into a band of adventurers that wish to raid a dragon's hoard (I don't remember if they think the dragon is dead or if they fully know the dragon is alive and are willing to fight it). Through the adventures, our protagonist learns he can use magic and actually uses it to help defeat a six-legged ogre by incapacitating it until the sun rises, turning it to stone. Over the next few books, his magic power grows to the point of contesting a necromancer and barely defeating it. His magic abilities include turning into other objects, not just animals but also trees. However, whomever taught him how to transform also warned him when he transforms, he might not want to change back. I remember some advice from the instructor, "There is great freedom in an eagle, complete rest in a tree, and terrifying power in a dragon," or something along those lines. Trying to find the title to recommend to a friend and to finish the series myself.

The Adventurers Wanted series by M. L. Forman. The first book is Slathbog's Gold.

Becca's suggestion of Slathbog's Gold by M.L. Forman looks very promising.

Amazon's Preview:
https://www.amazon.com/Adventurers-Wa...

Quotes from the book:
- Alex stopped and looked back at the bookshop window he had just passed. It looked like the same bookshop he had walked by hundreds of times before, but there was something different about it today. Instead of the normal pile of dusty books in the window, there was a large, brightly painted sign with two words printed on it in large red letters

Adventurers Wanted

- "Excellent," said Arconn. "Shall we discuss the contract?"
... "As a first-time adventurer, you are entitled to one share in twenty of the primary treasure, once it's recovered," Thrang begin. "In addition, you may receive bonus treasure as the leader of our company sees fits.