I tend to know where, broadly, I want visit before getting a book. To decide on a country or region I will look online, watch tv travel shows, browse books in the library, look at pictures, talk to people I know and then decide on a place. I’ll then buy a book or three on that place.
If it’s a region the book can help me decide which towns to see. In that case, where I need to narrow down which hill towns to go to or something, I’ll read each section to see where my interest lies. Otherwise I’ll just read the sections on my destinations and maybe skim the others just in case it changes my mind.
Then I’ll read the listings in the book. Skimming ones I care less about (shopping, because it’s not my travel focus, nightlife if I’m traveling with my children). I especially focus on things to see, since that’s the core of a trip to me. I will also supplement with internet sources. First to confirm things like hours and ticket requirements - I bet this will be even more important post COVID! Also to find other options. For me this especially applies to restaurants. There are so many and often the places I like best are not the more traditional things favored by many guidebooks. So I dig into food blogs and online sources. In general if you have a particular special interest a guidebook might not hit it so supplement with online sources.
Also I read the logistic sections closely shortly before I go. Things like how the subway works, etiquette, language phrases. That’s just very helpful if not super exciting.
But my most important tip is to MAP everything you are interested in, from the book, online, personal recommendations, everything. There are apps that let you put marks on your own places of interest on a city/country/world map, often you can color code to distinguish categories like restaurants/stores/museums. You save the map and can access it, add to it, whenever including when on your trip.
So the whole procedure can be reduced to read the book, come across a place of interest, confirm details online, put it on your personalized map.
Why is the map my #1 tip? It makes it so easy to plan and yet be flexible. You can see that museum A is a block from church B and a 3 minute walk from restaurant C, which is just off piazza D. There, you have a great, easy, logical day! And say you aren’t hungry so skip restaurant C and go walking in a random direction. Great, you walk for half an hour and are now hungry. Open the map, you happen to be near restaurant E and go there. Then you see you are only a block from a park you marked as having great gardens so off you go and so forth.