AceShowbiz - Michael J. Fox is ready to call time on his acting career.

In his new book, "No Time Like the Future", the 59-year-old actor, who is almost 30 years into his battle with Parkinson's disease, admits he's entering a "second retirement" and is ready for his acting career to end.

Sharing that his "work as an actor does not define me," Fox goes on to tell readers that his recent health struggles are only one factor in his decision to stop acting.

"The nascent diminishment in my ability to download words and repeat them verbatim is just the latest ripple in the pond," he writes. "There are reasons for my lapses in memorization - be they age, cognitive issues with the disease, distraction from the constant sensations of Parkinson's, or lack of sensation because of the spine - but I read it as a message, an indicator."

The "Back to the Future" star, who was diagnosed with the illness in 1991 at age 29, adds, "There is a time for everything, and my time of putting in a twelve-hour workday, and memorizing seven pages of dialogue, is best behind me... At least for now."

"In fairness to myself and to producers, directors, editors, and poor beleaguered script supervisors, not to mention actors who enjoy a little pace, I enter a second retirement," Fox writes. "That could change, because everything changes. But if this is the end of my acting career, so be it."

Amid his battle with Parkinson's disease, the actor found a noncancerous mass had been growing on his spine - threatening his mobility. As he learned to walk again following secret spinal surgery, he fell and broke his arm.

"It was when I questioned everything," he recalled. "Parkinson's, my back, my arm."