There is a booklet I wrote many years ago as a short but clear introduction to the use of financial ratios in the interpretation of financial statements. It is still useful* today although it is a bit dated in some respects.
It can be downloaded from here.
*This resource does not cover the interpretation of cash flow statements (see Week 3 materials and Seminar 2 material) or the interpretation of segmental information (see materials on interpretation for weeks 3 to 5).