'Choosing and using statistics: a biologist's guide' is one I used in my past.
https://sunsetridgemsbiology.wikispaces.com/file/view/Choosing+and+Using+Statistics.pdf
It's not free, but if using R then Rob Thomas guidebook is by far the best small reference book. I have a physical copy I always have with me when running analysis in R. Doesn't teach statistics per say, but very useful as a handbook whilst running statistics.
https://www.amazon.co.uk/Data-Analysis-statistical-Software-Scientists-ebook/dp/B014RIJBFA