Of the many online resources available for this topic, www.anaesthesiamcq.com (by Kerry Brandis) is the single most important reference. It is essential to become familiar with the entire contents of Brandis' Acid Base Physiology Chapters, at the very least with Chapters 3-10. The ABG interpretation questions in the fellowship exam can be navigated with moderate success without any other preparation.
Oh's Intensive Care Manual has a selection of chapters which act as the definitive reference for this topic; however, an indepth familiarity with the content of these chapters is not essential to the time-poor candidate.
The Oh's chapter by T.J. Morgan offers an excellent introduction to the quantitative physicochemical interpretation of acid-base disturbances. AcidBase.org is the official home of Stewart’s hypothesis. Unfortunately, only a vague familiarity with it is required.
Summaries and discussions of acid-base physiology topics from Life in the Fast Lane are made available as a list of links in the Resources section.