I am looking for books or other sources that focus on math that is above bachelor level (i.e. above just simple calculus). I am not looking for a specialized text for some field but just general mathematics at graduate level covering wide range of topic. My current math skills are very shaky after some simple univariate/multivariate calculus. If instead of textbook you know of good MOOC course that you could recommend it would help me as well. Thank you!
In my honest opinion the best book in this regard is fundamental methods of mathematical economics by Chiang and Wainwright.
It starts at the very basics and then goes all the way up to dynamic optimisation, also covering linear algebra, matrices, advanced calculus topics such as implicit functions and equation solving at all levels.
Combined with a fine and clear step by step writing style and lots of examples and exercises I find it really a good book.
If you have time and patience, "Foundations of Mathematical Economics" by Michael Carter is great. The book consists mostly of exercises that let you, broken down in manageable steps, prove many landmark results in mathematical economics on your own. Mathematics is not a spectator sport and this book will give you a great workout. If you do the work, you will come out with a very deep understanding not just of the relevant mathematics but a lot of topics in economic theory. Keep an eye on the erratum though.
If your goal is simply to learn the essentials you might need in grad school fast, "A First Course in Optimization Theory" by Rangarajan K. Sundaram is perfect for that.
Some good textbooks that focus on mathematics for economics are:
Essential Mathematics for Economic Analysis by Knut Sydsaeter, Peter J. Hammond, Andres Carvajal and Arne Strom - not technically graduate level but it has some topics that go beyond 'simple calculus'
Further Mathematics for Economic Analysis by Knut Sydsaeter - this is text that is fully on graduate level and it is actually a 'sequel' to the first book.
Mathematical Methods and Models for Economists by Angel de la Fuente - this book is a good first year graduate level math textbook - if you plan to just start graduate school this is quite a good well rounded source.
One which I have found to be a great level for someone who is slightly shaky in fundamentals is Carl P. Simon and Lawrence Blume's Mathematics For Economists. It is slightly pricey but has stood up as a main pillar IMO for mathematical concepts in economics.
The one for which I used in my undergrad and then used again in my preparation for grad school was Mathetics for Economics by Hoy, Stengos, Livernois, McKenna and Rees https://mitpress.mit.edu/books/mathematics-economics-third-edition. Perhaps on the cheaper side but just as full of content, it has lots of great problem sets within.
Hope this helps! Best of luck :)