Reddit reviews Problems of Knowledge: A Critical Introduction to Epistemology

Maybe try Robert Audi's or Michael William's introduction. They come at issues from somewhat different angles, but both are quite good.

Can we link to Amazon? FWIW, this book is a really clearly written--but not at all sophomoric--introduction to epistemology, which is a pretty central theme/issue in philosophy for lots of reasons. (Just in case the link doesn't work, it's Michael Williams' Critical Introduction to Epistemology)

https://www.amazon.com/Problems-Knowledge-Critical-Introduction-Epistemology/dp/0192892568/ref=sr_1_1?ie=UTF8&qid=1517609206&sr=8-1&keywords=michael+williams+epistemology

EDIT: I know you said you don't want "known philosopher's work" but I took that to mean the 'classics', so to speak, of which this book is not really one... but sorry if you were explicitly not looking for book recommendations!

I would recommend this book if you are just starting out (which you appear to be), and I really wouldn't advise starting with historical texts, they are fun but a text book of sorts does a much better job as a primer.

I don't see why you need more background in epistemology to get started on ethics (if you are worried you won't understand how philosophers come to conclusions in ethics, then you should pick up a book on philosophical methodology, e.g. "how to be a philosopher" books, not epistemology books). If you feel like you do, this book comes highly recommended (ernest sosa knows his stuff about knowing, and he liked it a lot):

http://www.amazon.com/Problems-Knowledge-Critical-Introduction-Epistemology/dp/0192892568

Since intuition comes up a lot in ethics, you may also like some of plantinga's stuff, e.g. Faith and Reason. Although some parts are technical, plantinga has a really fun way of showing how much of knowledge has nothing to do with inference or evidence.