What's the difference between this Great Britain book and either Rick's England guidebook or Rick's Best of England guidebook?
Rick Steves Great Britain covers Scotland, Wales, and our top picks of English destinations. Both Rick Steves Best of England and the complete Rick Steves England exclude Scotland and Wales, and just focus on England (with one exception: Best of England does cover Edinburgh). While the Best Of book covers fewer destinations in England than the Great Britain book, the complete Rick Steves England covers some English destinations not included in the Great Britain guidebook (Canterbury, Dover and southeast England, Brighton, Portsmouth, Dartmoor, Cornwall, Oxford, Blackpool, and some of North Yorkshire).
If you're going to Scotland as well as some of the destinations that are covered in our England books but not covered in the bigger Great Britain book, you could opt to forgo the Great Britain book and instead supplement your England book with Rick Steves Scotland. (If Edinburgh is your only stop in Scotland, you could instead grab Rick Steves Snapshot: Edinburgh.)
Should I get the London guidebook (or London Pocket guide) in addition to this Britain guidebook?
If you're spending only a few days in London, Rick Steves Great Britain is all you're likely to need.
If, however, you're spending four days or more in London, the extra information in Rick Steves London can be worthwhile. It offers more in-depth sightseeing information, several additional self-guided city walks and museum/sight tours, and more specifics on shopping and nightlife, plus activities for kids. Other information is generally the same — the London chapter of Rick Steves Great Britain has nearly all the hotels, restaurants, and nitty-gritty practical advice for London that you'll find in the Rick Steves London guidebook.
It would make less sense to supplement Rick Steves Great Britain with the smaller Pocket London guide, which includes just one walk and one museum tour more than what's included in the London chapter of the Rick Steves Great Britain guidebook, but otherwise offers far more limited coverage of the city's sights, hotels, and restaurants.