Hello! Which subjects, in particular, are you studying within this access course? I did not study history within my access to HE social sciences course, but I can suggest some psychology, politics, and sociology textbooks that I found useful for referencing in my assignments.
One general book I would recommend is the Palgrave Study Skills 'Cite Them Right' book if you are not familiar with referencing. Referencing correctly can easily improve your grade.
To add on to the suggestion of buying secondhand books, I was recommended to also look through the World of Books website for a more affordable option when I started my course. Also make sure to check your college's library resources, as they should in theory give you access to free ebooks relevant to your studies.