While I'll always vouch for Rough Guide's country guides, if you're after a City Guide the AA CityPacks are definitely the best place to turn to.
Practically, they are small and sturdy, with a good hard spine. They'll easily fit into any bag, even a woman's handbag, or even a pocket.
They also divide cities up into manageable chunks and provide good maps for each individual area, while enabling you to see how all the areas link up. As you'd expect, all the sightseeing or other recommendations in the book are also marked on the maps.
Nightlife, shopping and restaurants are covered in each section, along with tourist highlights, so you're whole day is pretty much covered. There are also additional sections on hotels, getting around, language, history and day trips from Budapest.
Most importantly, as far as my experience led me to discover, there are no factual errors. All the places actually still exist; are open and closed when the guide says; metro and other prices are correct and the summaries of things to do are to the point and on target.
The best guide book is the one you don't notice you have in the day - it's unobtrusive and doesn't take hours of searching through to find what you want - but that you feel the need to turn to of an evening or over breakfast in the hotel in order to plan for the following day. This guide fits that description perfectly.