I am a high school student searching for some mathematics books covering material all the way up to, but not including, college level mathematics. I have already read Gelfand's books and Lang's Basic Mathematics, and enjoyed them thoroughly. Yet I do not feel I have thoroughly mastered all facets of high school maths, and would like to do so before moving on. Are there any other books that it could be beneficial for me to study at this stage?
These books are not necessarily part of the high school curriculum, but I believe they really hel develop your mathematical maturity, which really smoothens the learning process.