There are actually multiple Robson guides. The second edition of the full version of "Birds of Southeast Asia" (2009?) actually has the most up-to-date taxonomy of any Southeast Asian bird guide. But it is bulky.
There is also a more portable field guide version of Robson's Southeast Asian guide (2005), which is not as up-to-date and may be what you are referencing. But for field use in Thailand I would actually recommend the 2002 Robson guide to "The Birds of Thailand." It has range maps and usefully limits the birds you have to consider when birding in Thailand. The taxonomy is somewhat dated, but at least for Phylloscopus warblers, most of the recent splits are virtually cryptic species that can only be reliably distinguished by vocalizations, so field guide descriptions are not that useful anyway.
Finally, I would recommend Lekagul/Round "A Guide to the Birds of Thailand" (1991 English edition is out of print) as a supplement to Robson. For one thing, it makes more of an effort to help with identification of Phylloscopus warblers, e.g. dividing them into groups. It also has descriptions of the status of each species in Thailand, which Robson lacks. However, as it is only available used, it can be pricey.
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