Operating System Concepts by Silberschatz and Galvin, how much down the edition timeline can I go to safely understand the core-concept
I don't know this book in particular, but I do know Computer Science text books in general, and offer this advice:
Consider the title: Operating System Concepts. It is about basic and fundamental concepts that underlie operating systems. The basic concepts, the core material of the text, are not going to change much from first edition to the tenth.
You are not being asked to study the text in such detail that you would be able to discover the differences between versions.
Many computer science books do get out of date. If the book was a user manual for Windows or a iMac then having an up to date one might be important, but I suspect not in this case.