The "additional terms may apply” piece dates back when Wikipedia was solely under a GFDL license (note that now text is under a CC BY-SA 3.0 License plus -almost all of it- GFDL). The GFDL has some less-common terms that would need to be taken into account on certain cases. For example, on a printed book, you would need to include a copy of the license, instead of just including a link, as with online documents. I find the additional terms may apply phrase a reminder to check the fine print of the applicable licenses.
Regarding possible pitfalls, first of all, I would recommend you to be wary of any image hosted on English Wikipedia. You can expect most of them to have been uploaded under a Fair Use rationale, (free ones would typically have been moved / uploaded directly to Wikimedia Commons). Thus, for any image you would be including, you should click on it and check the stated license. Plus, I'd recommend some checking that it actually makes sense and it was not a complete lie by the uploaded.
Even if the usage is fine from a copyright point of view, they may be protected trademarks or fail under different terms under your own jurisdiction (your book will be published in the US, and Wikipedia should be compliant with US law, though). Also, in some corner cases their criteria may differ. Particularly, there may be some files whose copyright was restored by URAA which were not detected as problematic.
Second, you shall credit every author. The copyright does not belong to "Wikipedia" (that's not even a legal entity), nor does it belong to Wikimedia Foundation Inc. It belongs to each and every contributor that edited that page. So, if you are doing things seriously, you would credit each of those that created the content you copy.
You can view a list of the people (and bots) that edited any page (and what they changed) on the history page. The link there to Page statistics would provide you a list of every user that edited that page. This would most of the time be a superset of everyone that would need to be credited (for instance on many articles there will users that added a vandalism, then immediately reverted. There's no need to credit changes that didn't impact the text you are copying). Also, if you only copied a section, you wouldn't need to credit people that only did changes on other parts of the page.
As a possible pitfall, in some cases text is copied which appears as added by one person, but was sourced on another page or different Wikipedia version, with a note on the edit summary. The authors of that prior work could easily go unnoticed.
As you are actually interested in talk pages, your task will be easier than for an article, as the different messages on talk pages should be signed by their respective authors.
As you mention, on those talk pages, users may have quoted other texts which are not under a Creative Commons license. It'd probably still be fine for you to Include those quotations in your work, but you should check anyway what you are going to include, their extend, if they are reasonable, etc.
Third, source the information. Rather than including the text and just saying "Copied from wikipedia", it'd be much better to say "Copied from talk page of 'UFO landings' on English Wikipedia as of 3 November 2020 - https://en.wikipedia.org/w/index.php?title=Talk:UFO_landings&oldid=986784877".
Fourth, note the license. Licenses have requisites, you should get familiar with everything it states (in the long, boring text!) and comply with it. For example, Creative Commons licenses have a requisite that you would have to include a link to the license itself, which is friendlier than having to copy the full text, as GFDL does, but still something easy to skip. If you also include images, note they may be under a different license than the text, so you would need to comply with those additional license terms with regards to that image.
Consult a lawyer as needed. Your editor may also provide you with some legal support. A printed book has a persistence different than online text. You wouldn't want to be put in a position to recall the first edition of a book due to a tiny legal mishap. Even more than in other mediums, it pays to do things right.
Finally, consider if you want to tell about your book to the people whose content you are including. While you are not required to do it, it's a polite thing to do, as they will probably like to know their work being used elsewhere. While I would probably not contact everybody (unless it was a very small set), I would recommend contacting the most relevant editors you are copying. Some might express a preference to be credited differently in your book, could provide interesting insights / be useful beta-readers, or interested to buy your book once it's in print. Expect that some of them won't reply or even read your message (they may have moved to other things, be uninterested, or even died). Note that from a logged-in account (with an associated email), it is possible to send other users an email by using a link on their user page (unless they disabled that option / don't have an email set).
Disclaimer: I am not a lawyer. This is not legal advice. Any mistake you commit by following random advice by a stranger on the internet is your fault. Good luck writing your book. :)