For history I wouldn't worry since it's a research based subject, textbooks are not going to do **** when you're looking for historiography in your essays or writing IA's etc. Textbooks are only a summary guide of sorts and you need to explore far deeper to get the type of grades required to achieve a band 6 or 7. Specialist books are required for research when you're writing your essays, generally you need to go to your state library since your local library is most likely going to be pretty useless (or whatever you call it in the UK). To give you an idea I go to the richest private school in my state (before you call me a rich snob, I'm on a scholarship and still need to live in a 2 bedroom flat with my family to afford it) and that still doesn't cut the mustard. I'm assuming you're jumping from GCSE to IB, this is a huge step up in standard of work required (e.g. for your IA it's expected to be university standard) and you will need to reference widely and with depth (you will realise how terrible wikipedia is once you start IB). So yeah textbooks don't mean ****, the official IB companion is okay but I never really use mine, the summaries don't offer any depth.
For philosophy and most group 3's (arts) you will need to go far beyond your textbook. I do psych as my second group 3 and there are about 4 display books worth of studies. And yes further reading will help, and in fact it is essential. Philosophy is a really really awesome course, I sit in during my spares whenever there's a TOK relevant class. I think you'll really enjoy it. History is the most rigorous arts subject in the IB so be prepared to do lots of work.
Also if you haven't made your decision to do IB yet, make sure you find out everything about it before you commit. It is much tougher than A levels and doesn't get recognised in the UK and I feel the majority of people would benefit from avoiding it. It really is a hell of a lot of work. My school doesn't allow anyone with anything less than B+ across the board and at least 2 A's in the subjects they look to continue, I'm not sure if it is the same at your school. In a sense it is a marketing decision to make our IB grades look amazing since it is a huge draw at my school but I also think it reflects just how tough it is.
These are two hard and grueling years. Good luck to you should you decide to do the IB