As you are being prudent, I'll assume you're on a tight budget (or maybe a Yorkie! )
Physics and our understanding of them changes little...so Principles of Flight, likewise, engine and airframe, met. change very, very little. Principles of navigation have also changed very little, probably Harrisson's accurate chronometer was the last major breakthrough. GPS is, AFAIK, still ignored for training- purposes, so compass, stopwatch (kitchen-timer) whizzwheel and ruler, it is.
Human performance is a bit fluid, as our understanding of the human body improves, but an"oldish" textbook is unlikely to lead you significantly astray.
Flight- planning is also pretty stable.....learn to jump through the hoops with the "manual" equipment as above and a current chart( which, of course, is out -of-date before it's even printed
) ,Then, AIUI, after you pass your Skills Test, you chuck it all in a dark cupboard (or flog it on fleabay, the Forum or AFORS to another stude , buy a "tablet" or use a smartphone, run Sky Demon and Bob's a close relative
said, that, really, just leaves Air Law, - another one that's like a chart (obsolete before printing!) but, look on the bright-side, unless you're very unlucky and there's been an overhaul shortly before you sit an exam, it will be as out of date as the study -material.
Note! I'm not a qualified pilot, the above is probably worth what you paid for it!
edit: check the current secondhand value against Amazon and ebay, I lent out book 1 of my ancient set of Trevor Thom's and it never came home....recently replaced for about a fiver (new name, new cover, same contents!)