If I can add something here. Principles of Physics and Fundamentals of Physics are the exact same books, just different regions of release. I can, however, explain how they differ:
I have seen both copies of the same editions of Principles of Physics, and Fundamentals of Physics. Fundamentals of Physics is usually found as the extended version, which also, compared to Principles of Physics, is slightly larger in scope (not by much) and has a lot more questions, too - close to twice as many questions; but, the text is pretty much word-for-word.
However, one thing, which is good news if you're in a region which you can only get hold of Principles of Physics (the UK for example) is that they barely differ from edition to edition. They are literally worded the exact same, and the only difference is that the questions may change, be re-ordered, along with maybe adding a couple of simple features, such as a learning checklist, etc.
A look on Amazon, and checking out the sample copy, will usually let you know what's new in that edition. But, in my opinion, it's barely anything, and not worth paying the extra - I bought the 9th Edition of Principles of Physics for £4.89 on eBay...DELIVERED! The 11th Edition would have set me back around £40 second-hand at the time.
You'll find it relatively easy to get an older edition, but my advice is that the amount it costs tapers off after a couple editions earier. I could have got the 8th Edition for £3.99, so was worth spending 90p extra for the 9th Edition. The 10th was £20. Gives you an idea of how far back to go, before the savings don't really matter.
But, just to add: the 9th Edition I've got is Principles of Physics Extended Edition, so it is exactly the same as Fundamentals of Physics. The only difference I've noticed, is that there are more questions than than the standard version of Principles of Physics, but less that Fundamentals of Physics. Fundamentals Extended seems to be their flagship book.
To give you an estimated idea, Principles will have around, on average 70 questions end-of-chapter questions, Principles Extended (my 9th Edition) has around 100, and Fundamentals Extended can have 120+. But, 100 is plenty!
If you are in the UK, or a region that is mainly where Principles is sold, then I would highly recommend to get the 9th Extended Edition. Just to add, this edition also features the level of difficulty of the questions, by dots, which I don't believe the standard Principles has. So, in a sense, Principles 9th Extended, is pretty much like Fundamnetals in terms of the content. Also, the author wrote a book called "The Flying Circle of Physics" (best of Googling this) which is mentioned at the beginning of Fundamnetals, and features parts of it throughout the book. Principles Standard doesn't have this, but my 9th Extended does.
Just to add to Muu9's comments:
I think the Principles of Physics text has these changes:
- No difficulty levels of problems indicated by dots (Principles 9th Extended has this)
- Cheaper cost (Definitely!)
- Black and white (Defintely in colour, for both standard and extended)
- Lower print quality (I'd definitely say the same, or at least mine is)
- No "flying circus of physics" excerpts indicated by an aeroplane (Principles 9th Extended has this)
- No student Solutions Manual ((Principles 9th Extended has this, and you can find it online in places)
- No GO(online solutions) (I'll have to get back to you on this one)
- And no Wiley plus (I've definitely accessed it with mine, but required me to input my university name, but it didn't ask me to verify. You can essentially add a course, click your course ID isn't listed, then just manually pick. This is all I did for my school and course. That's all it asked. This is how I've managed to "almost" thoroughly compare Fundamentals (this is the online ebook featured) to my Principles, and the Standard version. I actually use this for the extra questions if needed). I've just checked if my login still works, and it does. I never actually realised, but Fundamentals is actually on the 12th Edition now.)
Anyway, I hope this has helped? :)