I think you and your DC need to go into Smiths and look at them, they have very different look and feel.
e.g. Letts Science looks very different to the CGP? one. But the Letts covers all the boards whereas you can get some board specific ones which don't have non-required information. I and my DD find the CGP ones a bit 'busy' too much highlighting and fancy writing, but other people think nthey are great.
Some subjects they will have a text book for. I have actually bought the RE textbook as it seems good and there didn't seem to be a revision guide for the syllabus.
Some guides also include practice questions and exam tips.
For maths you can get workbooks with loads of practice questions.
Make sure you have a list of boards and if necessary the syllabus. No point buying a guide for 20th C History if your DC is doing Medicine Through Time and the American West!
DD in y10. So far we have needed guides for Science, (Maths), History, English. Haven't yet got languages as she actually has a textbook for these. Can't find one for drama anyway.
It would be impossible for DD to revise science without her guides.