Also worth mentioning is Peter Norvig's and Kent Pitman's style guide. It's got a bunch of examples but I've found the presentation helpful when thinking about style in general for other languages, too: https://www.cs.umd.edu/~nau/cmsc421/norvig-lisp-style.pdf
The code in Norvig's book "Paradigms of Artificial Intelligence Programming" is enjoyable to read as well. As a pdf the book itself is freely available: https://github.com/norvig/paip-lisp
And then of course there's the classic Costanza guide at http://www.p-cos.net/lisp/guide.html (not so much about style per se) that contains an important remark, "There cannot be "the one definitive guide" to Lisp. What I present here is what I would have liked to have seen in the first place. Other people probably prefer a different approach."
A quick glance at the broader open source ecosystem will show anyone that most projects don't follow this particular style guide, or this guide is silent on certain things that are probably considered good form enough to put in a company guide, like using uninterned symbols for packages, or using qlot for version pinning your dependencies and having reproducible builds in your CI system, or using UIOP for anything to do with file paths, or... One needs to get comfortable loading a system and then using the usual cross-referencing features of an editor (emacs, vim, atom (https://atom.io/packages/slima), really anything that can talk to a swank server...) since reading code will be the surest way to understand it, especially when it might not be laid out as you expect.