Complex sentence: This was regularly updated and reissued in revised editions (the last being the 11th edition of 1999), and became a standard resource for researchers: it has now been superseded by the online ARCHON directory maintained by The National Archives.
Split into simpler sentences: This was regularly updated and reissued in revised editions, and became a standard resource for researchers. The last edition was the 11th, published in 1999: it has now been superseded by the online ARCHON directory maintained by The National Archives.