My copy of 梵字必携―書写と解読 has just arrived. I bought it off Ebay which is the first success I've had with that website! (though the cover is much plainer than this Amazon image) The Google translation of the title is: "Sanskrit manual - inscription and decryption". Having seen the monumental Bonji Taikan ( 梵字大鑑) one can see that this is simply a cut down version of that book - more or less identical but less elaborate. This makes it perfect for me as I don't read Japanese anyway and I'm more interested in the pictures of Siddhaṃ characters.
I'd recommend this to the serious student - it has a complete syllabary, lots of conjuncts, some complex bīja, mantras, and a complete Heart Sūtra with a calligraphic commentary showing alternate letter forms.