Electronic resources are great. I use them heavily. But... Only a fool would rely entirely on having them available 100% of the time.
Connectivity is not assured everywhere - even if you have a local SIM and buy/have access to the local cellular network, coverage is spotty, and there absolutely will be times you can't get online. If you're counting on free wifi, don't count on that - it won't be there when you need it most. Electronics have batteries that get drained. Devices (and their special charging cables) fail/break/don't always work. Devices get lost/stolen. All of these things have happened to me on trips.
My philosophy is to have multiple, redundant/independent systems.
My wife and I travel as a couple. We each have an iPhone and a small iPad Mini tablet. Every document, app, map, saved file, and other detail - if its in electronic form, it goes on at least both iPads (critical items go on both phones, too). I generate a PDF version and ensure the stack of PDFs are on all devices and are readable before we leave. And every single one of those documents also gets printed out on paper, two copies: one for me to carry, one for her.
Note: This may sound like a lot of work but it's really not. As I do the planning for any trip (over many months) I have a folder on my computer for everything related to that trip; inside that there's a folder called "docs to bring". Every item goes in there (flight e-tickets, hotel reservation confirmations, tips from websites, info, etc.). Day before we leave, I go through that folder checking to ensure all the info is there and then print out all of it and move electronic copies to the iPads/iPhones.
Having literally every detail we would ever need about our entire trip in one, quick-and-easy-to-find place simply is liberating and removes 90% of the stress of travel. Everything is available anytime in seconds. We both have instant access to every tiny detail we might ever need...it's blissful. And all printed docs go in "the pouch" - a soft-cover portfolio - this portfolio is always in our day-packs, never out of reach.
Maps: We now always travel with a GPS (we have one with maps of all of Europe; for trips to Asia and Central America, I buy the separate map coverage and install it as needed). And we have the iPads with (fingers crossed) maps and apps. But we always, and I do mean ALWAYS also have a good quality, paper map (or foreign road atlas) purchased before leaving home. On a previous trip, my GPS was stolen (pick-pocketed in Lisbon, on the 2nd day of our trip). I had not had a paper map on that trip and it was a nightmare to try and drive around Iberia without it (and I discovered how hard it is to buy a decent map in most places). Lesson learned: always have a good paper map, no matter how well connected you are. Your gee-whiz devices, with all their convenience and amazing cool-ness, will fail someday. Count on it, and be ready for that.
Same for travel books: we get the electronic versions, but also take at least one printed book. Yes, books are heavy and bulky but sometimes you need the physical book. For most trips, I take the book to a copy shop, have them slice off the binding, I pull the pages we want to take, then they re-bind the slimmed-down book (with original covers) using plastic "comb" binding. This actually improves the book: it's slim and light, and best of all, it lays flat when opened. I once was in a cafe in Europe reading my book, and another Ricknic came over and wanted to buy my "deluxe" book from me (his didn't sit flat when open).
All the electronic resources are wonderful. But you can not rely on them to always be there when you need them. Gotta have backups/alternatives, because one day you're going to need them. Don't be like me, wandering around small Portuguese beach towns, lost and searching in vain for a road atlas or decent Michelin map (never found one). Paper is good.
Hope that helps.
