Openstax created by Rice University offer free text books in a range of subjects at no cost for digital copies.
It is possible to buy physical if you desire too.
It will probably not act a replacement for your Campbells Biology or equivalent, but for a free reference they are very good.
I have personally used Physics, algebra and calculus during my undergrad studies and well.. Principles are identical regardless of source after all.
These can also be' bought' for free on amazon if you search for 'openstax'
If you are feeling generous they also accept donations.
Hope it is of some use if anyone was not aware of their existence!