What elements do we need for an information-rich country field guide that is optimal for Google?

Great question, I hope I can be of help to you. My name is Humberto Valle, I have been working under my Agency, www.Unthink.me for about 10 years doing inbound marketing and advertising - a huge % of that is on SEO itself. With that said, I have actually worked with an European travel agency on this matter before.

First off, I want to clarify that I don't think Google 'likes' long rich text per page for example, their approach is much more complex than that - have you ever visited pages with minimal content but their ranking is impressive? In my experience the quantity is not as important as the quality.

In SEO - quality = targeted.
As far as your on-page SEO, my biggest concern is just how good is your targeted key wording? Are you using the right verbiage, language and jargon or descriptions for these areas and attractions as your would be visitors? You can have a lot of text but lack visibility if you don't use the same words as your consumers.

With that said, the way your sentences are 'read/structured' has a lot to do with SEO as well. Are your sentences clear enough that a robot computer could read and easily understand the definition? Google can now determine if you are referring about speed if you write the word fast and have the word car in the same sentence so with enough links and keywords your page could show up for the search "speed" even if you never include that word in your content.

Talking about long tail keywords - make sure to have strategic out links to other reliable, well established websites (not your own) that can also add value to the consumer... such as if you refer to a statistic about a certain region, make sure you write that sentence well and link it to the source of that data.

Make sure that your META includes 1-5 of your primary keywords (you should have anywhere from 5-15 keywords for a structure like yours) on top of your obvious targeted areas and attractions you are writing about.

Your URL extension should include the same title as the title of your page, i.e: Best Travel Destinations in Lisbon For Families With Small Children should have an url like :
www.mytravelwebsite.com/best-travel-destinations-in-lisbon-for-families-with...

Also try to break down the title through your sub categories (H2 titles) - this reiterates your topic to the search engines as well as remind your readers why they initially opened your link.
So a category could be :
Most Child Friendly Restaurant In X City <-- includes keyword 'child' and 'friendly' for example if they were part of my keywords.

Another thing you could consider is having something like:
" Related Article: www.mynextarticlelink.com "
somewhere towards the bottom of your pages... this yields more visit duration time which improves your SEO as well (the longer the visits the more trustworthy and valuable your site is)

Overall, Google maps everything in and out of your site and credits you for the value to create for the visitors. The more value to your targeted audience the higher your ranking and visibility will be. And just like in business, you can't please everyone at once, with SEO you shouldn't try to be found by everyone all the time or your SEO work will dilute.

If you would like some help, please don't hesitate to contact me or my team. We are known as the most helpful and affordable digital marketing agency in the world and we use Hubspot for ourselves and our clients.

Do not forget to include a lot of photos and graphics, and be sure to tag each one appropriately. This is another important and useful strategy that is "rewarded" by Google.

I think your site structure here will be essential in whether this project is successful or not.

Without knowing all of the details/just odd the top of my head, I would recommend breaking down your guides into location silos, something like site.com/country/field-guide-item where each field guide item is the topic you're discussing, such as "best restaurants in XYZ country" or "top water activities in XYZ country," making sure that each field guide item is based on keyword research for the topic at hand.

From there, you need to make sure you have optimized:
-Titles
-Meta descriptions
-H tags (as you've already mentioned)
-Images
-Breadcrumbs with schema markup
-Internal linking within the country field guide silo

I would not combine multiple areas as this will confuse google about what each page is about, so follow your gut there. If there is only enough to do in an area to write 500 words, then so be it!

I hope that helps as its based on not knowing your site and structure plan. However this should be able to get you started in the right direction. :)