For best plates and descriptions - Helm's Field Guide to Birds of East Asia (Brazil) - however I'm afraid the range of this guide ends just north of HK - so there are no specific mentions of HK with regard to specific species status etc - but it must contain nearly all the birds found in this area. We find this guide useful in Sichuan - and that's a lot further out of its range.
Of course where we are so many species are not included in the Helm's guide that we use it in tandem with the venerated but much berated - A Field Guide to the Birds of China (MacKinnon and Phillips).
Many things let this book down - most noticeably the plates, and distribution maps - which sometimes are a bit misleading to birders that are new to the area. But saying that - we'd be lost without this mammoth piece of work .
For travel lightness,the cost of only having to buy one book and better quality plates - I'd go for the Helm's - and use online sites if by any chance you stumbled onto a bird not included in the book.
PS that Helm's book also has its critics - especially for its rather 'contemporary' use of new common bird names - and making splits into species. This could be potentially confusing for newcomers to the area - but shouldn't be too much trouble for experienced birders.
"The Birds of Hong Kong and South China" by Viney, Phillipps and Lam is the standard guide for HK. Its pretty good, if not perfect, but certainly better, and substantially lighter than Mackinnon, which is confusing for offering you the full range of species from across China.
Brazil does not cover Guangdong, but is newer, and certainly covers all the palearctic migrants, but species that rarely or never occur in Japan are treated with less accuracy.
However, If its just for a few days the HK guide is perfectly adequate.