STEM learning has an extensive library of free resources for Mathematics (and also for Computing, Design and Technology and Science) including textbooks; you can search the collection in various ways, a search for textbooks for ages 11-18 returns these results.
Note that you can apply filters, so you could choose to look at a more limited age range such as 11-14.
To highlight some of these resources (images link to the STEM learning resource:
By David Kirby and Peter Patilla we have Maths Investigations and accompanying comprehensive teachers’ guide which contains 80 cards based on 10 themes.
Lorraine Mottershead’s book: Investigations includes many puzzles and investigations; solutions are also provided.
Also availanle from the same author is Sources of Mathematical Discovery.
Look at the Integrated Mathematics GCSE texts for example. Many texts are available from the Integrated Mathematics Scheme, the Year 8 book for example includes Basic probability and two units covering the basic notation of sets and Venn diagrams. Marjoram’s Exercises in Modern Mathematics is more suitable for able students, the Matrices chapter could be useful for those study Further Mathematics.
A Nelson Thornes series for GCSE Maths is available which was developed for the days of the three tier GCSE Mathematics. These are attractive in their presentation and could be useful for projecting examples.
Other titles which caught my eye include:
- Nuffield National Curriculum Mathematics
- Spode Group – The Problem-Solving pack
- Spode Group Resources
- Motivating A Level Mathematics – The Spode Group
- Nuffield Advanced Mathematics
- Extended Tasks for GCSE Mathematics