Translation of Weber's Lehrbuch der Algebra vol 1, 2, 3

This is a very deep subject which continues to be pivotal in contemporary number theory. I've never read Weber carefully, but my impression is that his arguments are difficult to follow and regarded as somewhat incomplete. For example, in his solution of the class number one problem, Heegner cited some of Weber's results, and my sense is that one of the reasons that people originally rejected Heegner's argument was that these resuls of Weber were regarded as unproved.

There is a survey by Brian Birch from the late 60s where he goes over the results of Weber that Heegner uses and shows why they are all valid (and hence why Heegner's argument is valid); his arguments use class field theory.

In general, I'm not sure that you can learn all that much about this subject without learning some Galois theory and algebraic number theory; indeed, proving the relationships between quad. imag. fields and elliptic functions was one of the driving forces in the invention of algebraic number theory and class field theory.

You could try the book of Cox, Primes of the form $x^2 + n y^2$, which surveys some of this material. I would guess that it uses more algebraic number theory than you would be comfortable with, but perhaps it will give you some hints.

There is also the book $\pi$ and the AGM by the Borwein brothers, which gives a very eclectic survey of some this material. The proofs are both elementary and (often) quite unusual from a modern, systematic point of view. But they may be more accessible to you, and it is an amazing book with a lot packed into it.

There apparently is no English translation, but there is a French translation:

Traité d'algebre supérieure

Author: Heinrich Weber Publisher: Paris : Gauthier-Villars, 1898.

A translation of the original might not exist as such. However edification might be found in alternate sources. Some books that might of interest to you are:

A. Weil, Elliptic Functions According to Eisenstein and Kronecker.

I do not consider Weil to be the best introduction; but in principle you should be able to read this book with some background in complex analysis/algebra.

The other book that might interest you is

G. Shimura, Introduction to the arithmetic theory of automorphic functions.

Again I do not know if this is the most optimal book; for example the author uses a very old language for algebraic geometry. But in principle you will be able to read it. At least at the start.

Much more accessible than Shimura will be the last chapter of the book,

J.-P. Serre, A course in Arithmetic.

Another very relevant reference is S. G. Vladut's "Kronecker's Jugendtraum and Modular Functions". Parts of this may be too advanced for your present state of knowledge, unfortunately. Still you might find some introductory parts advantageous.