Depends on what you're looking for.
General introductory books transitioning to degree level are:
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What is Mathematics?. This covers some basic set and number theory, geometry and analysis. Nothing is covered in a lot of depth but its very well written. The last chapter is now a little out of date (in places) but even so is still a good read
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A Concise Introduction to Pure Mathematics. This covers the foundation part of most first year courses, covering again basic set and number theory but in a very different style to What is Mathematics?
I would spend some time reading through
this course guide, which give a brief description of the sort of thing you do at degree level. A lot of it is not about solving complicated equations but about understanding the mathematics you already know from A-Level. It is a lot more rigorous which might take some getting used to.
Once you have a feel for what you want to pursue, there are lecture notes aplenty around the internet. From a simple search:
Differential EquationsProbabilityAnalysisI purposefully chose three different institutions as there are loads of lecture notes around - e.g. see
Cambridge (I'm sure there are better examples of Cambridge notes) and
Oxford examples
The quality of notes is not dependent on the reputation of the university. As I was studying I used lectures notes from all over the world, just keep trying until you find something that works.
American universities have a different style of course but again have some excellent notes.
If you're looking for questions, at university they're called problem sheets or example sheets. Most university courses have these public, even if the notes aren't so there will be lots for you to choose from.
Finally, if you're at university doing Medicine I'm sure you can browse their maths books until you find something that gets your interest.
(Specific examples of books/notes will be easier once you've narrowed your interests a bit)