Some of these things may have changed, but this is how it's been for the past few years:
MATH1141: There's a
thread with useful information here. You won't be able to get an electronic version of the textbook because it's printed at UNSW each year. Don't worry, it's cheap (well, relatively, around $45) but very very good! The other prescribed textbook you generally won't need. The Higher course is significantly more difficult than the lower, but the grades scale up quite nicely.
PHYS1131: I found that you can get through the course without the textbook quite easily. However, the textbook is very good for the follow on course
PHYS1231, but you can find it easily online anyway. Unlike for Maths, the higher course is exactly the same as the lower course, except you do more questions in the final exam. Higher grades scale up quite nicely too. I recommend doing Higher even if you think you might not be able to.
COMP1911: A much better choice than the "Computing for Engineers" course, ENGG1811. It's very breezy if you have no programming experience at all or even know what programming means. If you do,
take the higher course, COMP1917. It is a much better introduction to computing overall. If you know any programming at all it should be quite a simple course. If you don't, some people tend to find it difficult. The textbook is completely unnecessary as programming is something you can find an abundance of resources on the web for. However, if you really struggle, consider buying it.
ENGG1000: While you don't need the textbook (it is completely useless), when I did the course we were required to write a report, and the report has to have a bunch of useless BS from the textbook (if you want to get a HD in the course at all). Find a copy in the library and photocopy the relevant pages, and you'll have all you need.
