Having never seen Langrand et al. (1990), I'll say this is one of the best guides for Madagascar and indeed one of the best around for any specific region. It is that rare sort of guide where every effort is made for it to be comprehensive; even though it is photographic, any birds for which a photograph is missing are still illustrated with an excellent painting. The choice of photographs, meanwhile, is superlative, good enough in 99% of cases to be diagnostic, with every effort made to track down the best possible shots of all the endemics.
Most photographic guides are all about the pictures, often crowbarring in an image that isn't necessarily diagnostic [probably partly to bulk up and partly out of the sense of achievement in actually having sourced A photo of a rare bird] and also not imparting much in the way of information. This book, however, doesn't forget what it set out to do, and as such has, in addition to a decent image, also provided excellent descriptions of all plumages, voice, behaviour, range and habitat notes, and best of all, notes for distinguishing a bird from its most similar relatives and recommendations of where in Madagascar to go for each species.
With no new species described since this book's publication and only one omission [the Bluntschli's Vanga, whose taxonomy is highly dubious and no one has seen alive anyway], this book does not threaten to go out of date anytime soon. Madagascar's avifauna is relatively self-contained, with a manageable number of species for an exhaustive field guide to be possible; this book has made sure future guide writers have no excuse in not putting out a book of at least similar standard.