How do copyright laws affect the creation of teaching resources based on books by other authors?

I am looking to set up on online business which sells teaching resources for download (based in UK). These teaching resources will be based on giving teachers ideas and frameworks for teaching fiction texts in the classroom. The resource books may include short extracts from the books of other authors in order to illustrate certain teaching points. The resource books will also reference specific pages from the books of other authors. How will this be affected by copyright laws? What do I need to do to ensure I am not sued for copyright theft?