I am in the process of creating a personal collection of reference materials for various areas of mathematics, including statistics, combinatorics, calculus, and linear algebra, as well as others such as group theory, set theory, and topology.
The goal is to have a convenient, easily accessible resource for future projects and studies, rather than having to search through a large number of books to find specific properties and information. However, many of the textbooks I am currently using are quite lengthy and not structured in a way that is useful for this purpose.
I am looking for recommendations for texts that could serve as a foundation for my own set of documents. Ideally, these texts would be concise, and cover a range of difficulty levels, from introductory to advanced.
As a first-year student, I am particularly interested in refreshing my knowledge of linear regression and inferential statistics."
I give the Amazon links not to push you to buy (they are often too expensive, and if you can get them at your university library, go ahead!), but to have a look at tables of contents and reviews.
Statistics: Schervish, "Theory of Statistics". Probability would probably(!) be useful: Borovkov, "Probability Theory".
Forget calculus and learn analysis, once and for all :)
Real analysis: J. Yeh "Real Analysis : Theory of Measure and Integration" (covers Lebesgue's integral and a good deal of functional analysis).
Also in analysis: the two Rudin books, "Principles of Mathematical Analysis and "Real and Complex Analysis"
Groups and linear algebra: either Serge Lang "Algebra" or Dummit & Foote "Abstract Algebra". They may be too abstract (you will find Galois theory, modules, etc. in them), so if you are only looking after matrix theory, I could suggest Axler "Linear Algebra Done Right" (I don't own this one though, but I believe it's quite appreciated). If you need numerical linear algebra: Golub & Van Loan "Matrix Computations", Demmel "Applied Numerical Linear Algebra" or Trefethen & Bau "Numerical Linear Algebra"
Topology: Munkres, "Topology", but many texts are worth a look, even if they are not as known. I like Newman "Elements of the Topology of Plane Sets of Points" for instance, and Steen & Seebach "Counterexamples in Topology" is a jewel, to really understand the limits of many theorems.
You didn't mention complex analysis, numerical analysis, differential geometry, and many others, so I'll let you update the question if you feel the need.