OU = Open University. It's a UK distance learning university. Website is at https://www.open.ac.uk/

The courses the comments refer to are the level 1 courses listed here: http://www.open.ac.uk/courses/maths/all-modules

The text books are the texts for these courses. I don't know if you can get the books from OU without registering for the courses.

The OU; the Open University. It's a British university founded sixty or so years ago with the intent of widening access to higher education. Their campus is centred in Milton Keynes, a bit north of London.

I took a Masters in Maths with them and it was brutal. Serious exams are a youngster's game. I got a glimpse into why so many Cambridge wranglers were also serious atheletes.

The books are called "Essential Mathematics" 1 and 2. They correspond to these modules. http://www.openuniversity.edu/courses/modules/mst124 http://www.openuniversity.edu/courses/modules/mst125

Second-hand OU books are usually bought from https://www.universitybooksearch.co.uk/

Could you provide the direct links for the books used in: MST123, MST124, MST125? I could not find them in your links.

If you're not in the UK, I see that Thriftbooks appears to get at least some copies[1].

1: https://www.thriftbooks.com/browse/?b.search=MST124#b.s=most...

Search for "Open University MST124" or similar on www.ebay.co.uk or www.amazon.co.uk. There seem to be plenty of copies of 124 available on both at the time of writing.