My daughter's school uses "Impact Maths" Heinemann which is very comprehensive and has meant that we haven't needed to purchase any additional texts. ISBN 0-435-01758-6.
Can you post which area (s) your daughter has difficulty with - difficult to advise further without knowing. There are 4 areas of the maths curriculum :
Using and Applying Mathematics
Number and Algebra
Shape, Space and Measure,
If you're not certain as to which area needs support have a look at the BBC revision website - it is interactive and is split into sections so you can revise and test yourself too. www.bbc.co.uk/schools/ks3bitesize/maths/
Is your school covering totally new work from KS2 which is causing problems or is it the pace that is the problem rather than the complexity of the work. Usually there is an overlap with KS2 initially, how was your daughter at Primary with her maths?
Also is your daughter totally secure in knowing her tables, and the four basic methods of addition, subtraction, long division and multiplication. If not this could slow her down considerably and hamper her progress. You may need to revise some of the KS2 work rather than looking at KS3 books.