It appears that one of the most important books written about Johann Tetzel is quite old, and never translated into English: Johann Tetzel der Ablakprediger (1899) by the Roman Catholic writer Nikolaus Paulus. I've been searching for a full-length contemporary and scholarly treatment of Tetzel in English, but I've yet to come across such a book. Most of the secondary literature I've found simply refers back to Paulus.
Having a translation of Paulus would be helpful, because I'm left further away from Tetzel by relying on what other writers say what Paulus said about Tetzel. That being said, I did come across a Roman source that seeks to gives an overview of what Paulus compiled about Tetzel: Tetzel the Indulgence Preacher (1902) by John Corbett. The article is worth a read. You'll come across Reformation polemics in which some went too far against Tetzel, similar to the way some go to far against Luther. Yes the article has an underlying slant that the entire concept of the indulgence is Christian and that the St. Peter's building project was a worthwhile endeavor, but try to look past this to get a better understanding of how Rome's apologists defend Tetzel.
The basic gist of the article is that Tetzel consistently taught that a plenary indulgences was an entire remission of the temporal punishment for specific sins. The guilt of these sins had already been remitted by contrition and confession. In other words, Tetzel was teaching that someone seeking an indulgence had to be truly sorry for their sins, or else the indulgence would be of no benefit. The idea that future sins could be remitted by an indulgence was never taught by Tetzel, but was rather a false accusation brought on by Luther. If someone received an indulgence for sins and then immediately went out and committed the same sorts of sin again, well, it would then be time to go back and purchase another indulgence.
Tetzel comes under more scrutiny in regard to indulgences for the dead. He is charged with teaching speculative Roman doctrines as certain. He's said to be in error for holding the following as "Christian dogma": "In order to gain a plenary indulgence for a soul in purgatory, it is not necessary to be in the state of grace. It is enough to give the alms." This means that if one wanted to buy an indulgence for a deceased person, the holiness (or lack thereof) of the purchaser made no impact on the efficacy of the indulgence for the departed soul. Second, Tetzel held that "such indulgences are applied infallibly to the particular soul for whom they have been obtained." In other words, when Tetzel urged his hearers to have pity on their parents "suffering the most dreadful pains and tortures" in purgatory by buying them an indulgence, it wasn't certain that those parents were going to get the purchased relief.
One other sale of indulgences from Tetzel is worth mentioning: confession-letters or indulgence letters. These could be "procured without contrition." One need simply make the purchase. It appears that for these to actually work, the purchaser had "to choose a suitable confessor, from whom absolution could be obtained even for sins usually reserved to the Holy See." This particular purchase could be used two times: "once during life and again at the hour of death."
The speculative and curious part of the article says that perhaps the indulgence preachers were not careful as to how they presented this theology and it led to misunderstanding by the people- Tetzel though shouldn't be blamed for this. I can agree with this to an extent- that if Tetzel was preaching the indulgence doctrines as has been defined above and the people heard complete remission for all my sins (past, present, and future) without contrition or confession, and there isn't historical proof that Tetzel taught what was heard, then we should consider holding Tetzel responsible for what we know he did say. On the other hand, the article does point out via testimony that Tetzel "devised new ways of getting money... and finally there resulted scandal and contempt among the common people and censure of such spiritual treasure on account of the abuse." The writer summarizing Paulus reluctantly concludes that "...it would indicate on Tetzel's part some excess in urging the faithful to contribute." The article doesn't really explore this "part." Perhaps Paulus did. It would be interesting to read how Rome's defenders describe "some excess."
Overall, what the article demonstrated to me was that one can quibble about the historical facts of the Reformation, whereas the point of contention should actually be the theology of the indulgence. That's what appears to have happened to Luther as well- he began arguing over the indulgence issues of his day and then arrived at seeking to refute the ultimate theology behind the indulgence. Imagine living in the 16th Century and purchasing some incredible indulgence. The responsibility then to not waste your money by seeking holy living demonstrates a crucial difference between the Gospel of Christ's perfect work and those who seek to save themselves by their works and what they can afford.
Here's a negative contemporary review of Paulus from The Methodist Review. "Paulus himself confesses that, at least in substance, Tetzel taught that as soon as the indulgence fee rattled on the bottom of the money box the soul for which the indulgence was purchased sprang out of purgatory."
Here's an interesting tidbit about Paulus and the historicity of indulgences: