Philippine numismatics has been suffering from a scarcity of new publications for decades. This is truly unfortunate, and should be corrected with sound research, good writing, and a publisher who isn't in it for the money. On this last point, some of the most successful books in recent memory have been self-published by the authors or numismatic groups, rather than the big publishing houses.
In addition to "Filipino Numismatist's" excellent suggestions, Neil Shafer in the US has begun writing articles on Guerrilla notes for Bank Note Reporter. He is one of the most respected and prolific writers on Philippine numismatics, on both notes and coins. It is accurate to say that his articles are a serious update on his books on notes written over 30 years ago. Only Guerrilla notes distributed in a few provinces are being printed from what I have heard.
Stay tuned, because another publication on PhilNum will be appearing in the coming months.