No point buying textbooks but if you want to I can reccomend some. You can't buy a textbook for english since every school has a different syllabus but if you want to, you can find one that suits the things you'll cover in the year for english. For maths there are Cambridge, Insight, and Mathscape.
for maths i just use the 2u advanced textbook (cambridge covers all the topic thouroughly), it isn't a large step up from year 10 5.3 and much of it you have already covered at school. studying in year 10 for 2u won't be that hard as normally you have a lot less hw.