You might be interested in the book Graphs, Groups and Trees by John Meier. It is a very readable introduction to "geometric group theory", and is pitched at "advanced undergraduate" level (in the question linked in the comments, some of the books are graduate level and above). Geometric group theory can be interpreted as "the study of groups using their actions", and the simplest actions are actions on graphs. Hence, this book studies groups by using their actions on graphs.
Topics covered in the book include group actions, Cayley graphs (every group acts on a graph, and the Cayley graph is such a graph), actions on trees and basic Bass-Serre theory, the word problem for groups, regular lagnauges and normal form, and the coarse geometry of groups. There are lots of examples to get your teeth into (every other chapter takes a specific group and analyses it using the previous chapter).
I should say that Meier's book assumes a working knowledge of group theory, but I would be surprised if there existed a book on this subject which did not!
@ABajaj The book you were reading, by Grossman and Magnus, was from the "new mathematical library". This "library" was a collection of books pitched at your level (US high school) which accompanied a new method of teaching maths (called new math) in the US. The method was generally considered a failure, and therefore I would doubt if there was a set follow-on book. Meier's book perhaps requires a small jump from where you are just now, but this jump can most likely be helped by also buying an introductory group theory text to use as a reference. If you know what a normal subgroup is, then you are probably good to go! (The word "Sylow" does not enter Meier's book, although enters every single "standard" group theory text, so your jump will not be too big!)
I should point out that the name of Magnus is a famous one in geometric group theory. So Meier's book, and geometric group theory in general, is a natural place to go next.
Since you're looking for a book of an "introductory" level and which starts from the basics I think you should have a look at the book "A first course in Rings and Ideals" by David Burton.
This book covers the basics of ring theory, e.g., maximal and prime ideals, isomorphism theorems, divisibility theory in integral domains, etc; and also includes some topics of commutative algebra (chapter 12: "Further results on noetherian rings"), as well as, noncommutative algebra (chapter 13: "Some noncommutative theory").
On the other hand, because not only the theory is important, but also the practice in order to master a subject, I would suggest you to check the book "Exercises in Basic Ring Theory" by Grigore Calugareanu and Peter Hamburg. This book, as the title says, its aimed to help you to understand the basics of ring theory by offering a nice set of exercises from the fundamentals to rings of continuous functions. I recommend you to use this book as a companion of Burton's book.
Best Answer
Handbook on Statistical Distributions for Experimentalists - University of Stockholm
A link to the pdf http://www.stat.rice.edu/~dobelman/textfiles/DistributionsHandbook.pdf
Cambridge University posts lectures notes and homework online. You could visit there. There is a list of courses available here http://www.cl.cam.ac.uk/teaching/1415/
Transforming Density Functions - https://www.cl.cam.ac.uk/teaching/2003/Probability/prob11.pdf
Convultion of distributions
Here are some lecture notes I found from MATH 526 University of Washington Lecture 8 will focus upon convolution. This link will take you to all the lectures and homework.
http://www.math.washington.edu/~hart/m526/
http://www.math.washington.edu/~hart/m526/Lecture8.pdf
University of Manchester - Products and Convolutions of Gaussian Probability Density Functions
http://www.tina-vision.net/docs/memos/2003-003.pdf
If you could be more specific I am sure I could find more.