If I may contribute an opinion, I use Sibley and Stokes books, but It doesnt mean I consider any other of them bad. RE: the apps , Marilyn seems to be making excellent use of her Merlin app. I dont do apps.
If you do get a book , IMO you should check that it has representations with open wings , top and bottom , shows juvenile birds as well as both adult genders in both basic and alternate plumage .
( basic plumage is the off-season non-breeding plumage..... alternate is the showy version,,,, For birds like gulls which mature over three or four years they may look very different in each of those years.
Photos are the most natural thing to compare to, but they may not be self explanatory and may in fact be very misleading as to which characteristics really exemplify the species and differentiate them from others. (so you know what features you need to pay special attention to)
Hope that helps
Stosh