There is a huge range of books aiming to help children learn particular subjects or topics at given ages, Key Stages or levels of achievement. Start with your local library. Buying books for all of the topics you may wish to cover could end up costing a small fortune.
If you are aiming for your child to take particular qualifications (eg: GCSEs), make sure that books you choose match the syllabuses set-out by the relevant exam board. Most books such as revision / study guides will clearly indicate the syllabus and exam board they are aimed at. Also note that revision guides may not be sufficient on their own and supplementary material will probably be needed.
Other resources can be found on the Internet. You may need a little time and determination but you should be able to find free information, worksheets, quizzes, puzzles and ideas for projects etc for any NC subject (and lots besides) for all ages. Use detail in your web searches; eg: ‘maths worksheets fractions key stage 2’ and then sift through the sites listed. Keep a note of useful sites or of sites that may be of use in the future, perhaps add them to a ‘Favourites’ folder. You should obviously consider internet safety and further information on this can be found at www.stopitnow.org.uk.
The BBC website also contains a large amount of useful material. Scroll to the bottom of the BBC Homepage and you will find the 'Explore the BBC' section including everything from ancient history to the weather forecast. There are also links to 'Bitesize', another very useful free resource. Try www.bbc.co.uk/schools/parents/search/ as an easy to use search for resources.
Companies and Organisations
Many companies and organisations have very good online resources and some will send out free information / education packs on request (eg: the National Space Centre, Woodland Trust, UK Coal, BP, E.ON etc).
Look out for suitable TV programmes. There are a huge number of documentaries shown and many dramas (eg: period dramas) and 'sciencey' entertainment programmes can also be very useful. Obviously the usual caution is needed, some 'sciencey' shows can be misleading and anything but genuine Science and period dramas may stray from historical fact for plot value. Even if a show does contain accurate and pertinent information, it may have content you would prefer was not seen by your child.
Stuck for ideas?
If ever you and your child find yourselves stuck for ideas try a quick web search for today’s significant events, battles, historic figures’ birthdays, scientific discoveries made this day in 1723 etc. This could lead on to some sort of project or at least a little research.