Can’t believe no one has mentioned Vector Calculus, truly horrendous stuff... The ”university” maths book is by Riley, Hobson, and Bence, and is great... if you have a maths degree already. VC was even a problem for the mathematicians, ask them and they’d just shake their head.

Anyone looking for a science or engineering maths textbook, I can strongly recommend ”Advanced Engineering Mathematics” by Erwin Kreyszig. Covers all the bits you’ll ever need, and actually tells you where you might come across it, and also gives REAL examples, as opposed to the usual contrived ones in most books