There are two restaurants NOT in Rick's book that are definitely worth eating at in Budapest, particularly if you like the good traditional Hungarian fare (we liked it a lot). Both are in Lipotvaros (near Szabasbag Ter).
The first is a relatively new restaurant that I think was not open the last time the guidebook was updated - Hungarikum Bisztro, on Steindl Imre U. 13. The reviews on Trip Advisor do NOT exaggerate, but you MUST have a reservation (and book ahead; we started asking Monday and didn't get in until Thursday).
The second is a smaller place called Csarnok Vendeglo. It's in the Rough Guides book, located on Hold Utca 11. This place is a simple (like Lugas Etterem, Rick's recommended place), with wooden benches and ridiculously low prices (both had goulash and entrees, glasses of wine and a side for less than $30). Best goulash we had in Budapest (and we had it 3 times). Good, simple, delicious food. ("Finom" is the word for delicious in Hungarian; I would use it for both places).
Happy Eating in Budapest!