Why is "trabajar" being conjugated to the "yo" form?
The short answer is: no, "trabajar" is not being conjugated there. In fact, in "años de trabajo duro", "trabajo" is not a verb, it is a noun.
Here are a few examples of use of "trabajo" as a noun:
- Mi equipo aprecia mi trabajo (My team appreciates my work)
- María está en el trabajo (Mary is at work)
- El trabajo de oficina es aburrido (Office work is boring)
If you look up the definition of "trabajo" on wordreference.com, you'll find not only the uses listed above, but also the conjugation of the verb "trabajar", which is the cause of this misunderstanding:
Is the sentence trying to say "This great book is the result of years of my own hard work"?
No, it doesn't. The translation in your question is perfect:
Este gran libro es el resultado de años de trabajo duro.
This great book is the result of years of hard work.
It does say the book is the result of hard work, but it doesn't specify whose work.