There are a number of translations of Luther's "Bondage." Here's one freely online that's translated by Vaughan.
The Bondage of the Will by Luther translated by VAUGHAN
https://www.monergism.com/thethreshold/sdg/luther/Bondage%20of%20the%20Will%20-%20Martin%20Luther.pdf
OR
https://www.onthewing.org/user/Luther%20-%20Bondage%20of%20the%20Will%20-%20Vaughan%201823.pdf
UPDATE:
I first read Martin Luther's The Bondage of the Will [wiki article]
in [circa] 1998. The J.I. Packer and O.R. Johnston translation. Around
2004 I read Catholic theologian Harry McSorley's classic book Luther: Right or Wrong? Which is a review of Luther's book and which, in my opinion, every Protestant should read, along with Luther's Bondage and Erasmus' Freedom. Recently (Jan. 2020) I finally read Erasmus Desiderius' On the Freedom of the Will [wiki article] found in the The Library of Christian Classics edition Luther and Erasmus: Free Will and Salvation.
Translated by E. Gordon Rupp. Having finished Erasmus' book, I decided
to re-read Luther's work in the translation offered in the same edition
[translated by Philip S. Watson]. Luther's Bondage was in response to Erasmus' Freedom. Erasmus would latter respond to the Bondage in a two part book Hyperaspistes. However, the Hyperaspistes never gained any popular recognition.
I mention all of the above to preface my recommendation of Luther's Bondage.
I've personally read it, and highly recommend all of my fellow
Protestants read it as well. I realize that the book is by no means
perfect. There are modern theological and philosophical distinctions and
categories which show some of the gaps and weaknesses in Luther's
reasoning [cf. McSorley's book]. Nevertheless, Luther goes a long way in
defending and demonstrating God's sovereign unconditional election on
Scriptural grounds. Even McSorley's book praises much in the work. The Bondage truly is, as has been noted by others, the Reformation's 'Manifesto' on Sola Gratia.
While neither the Packer & Johnston or the Watson translations are
freely available online (being in copyright), Henry Cole's translation
is freely available (being in public domain).
Here's the book in:
HTML
PDF
AUDIOBOOK [LibriVox recording]
The LibriVox recording has also been uploaded on YouTube:
https://www.youtube.com/watch?v=7rMsUTLpm3M&list=PLzOwqed_gET0wH_DTW4SkxgFjd0PjrotE
