Is it rational to lose confidence in authors whose books contain many typos? I make mistakes too as a flawed human. Yet I always lose confidence in and doubt authors (whose native language is English) whose textbooks or journal articles contain glaring typos or orthographical errors. Is this expectation rational or warranted?

For instance, Michael Spivak's Calculus contains many such typos. But I've spotted no typos in David Benatar's books. Here are my assumptions:


The author or the publisher can hire or engage multiple proofreaders (who can be university students, friends, or family members) to proofread.
The publisher permits the author to read a draft of the book before it's printed, to spot any last-minute mistakes. So these typos aren't the publisher's fault.
Your assumptions are faulty. Unless a book is self published, it is the publisher only who hires reviewers, and, more importantly, editors. They are too expensive for the author to engage. A text that sells for about $90 USD returns about $5 USD to the author and sells between 0 and a few thousand copies (with a few exceptions). The cost of the textbook prep is all up-front other than reprinting.

For many (most?) people, proofreading your own work is nearly impossible. If you made an error when you first wrote it, it was because of some slip of the mindset. Thinking one thing and writing another. When you go to re-read it you actually read what you thought you wrote, not what you actually wrote. It is the publisher, and more important, the reviewers who should catch these since they read it with a fresh mind. The author isn't perfect but he/she is innocent here.

Some authors and publishers will pay bounties for errors caught by readers so that they can be corrected in future printing (and editions). Donald Knuth famously did this for The Art of Computer Programming. 

Many authors an/or publishers will also put errata lists online where readers should be able to find them. 

Typos are annoying, of course, and the other qualities of any given work have to be sufficient to overcome that annoyance. 

But it is the publisher of the book that pays the upfront costs of editing, review, typesetting, etc. They (publishers) earn a ton of money for this and are responsible when it goes badly. While reviewers often work for free, editors don't. They are employees of the publishers.