Who is William Ashbless? If you're asking this, you haven't yet read The Anubis Gates, in which this fictional poet/author is a central figure. So look there for a more complete answer to this one, but the fact that the character is a writer is the reason why references to him show up elsewhere.

"When Blaylock and I were in college in '72, the school paper published a lot of post-hippie crappy poetry, and we decided to write portentious, heavy-sounding poetry to make them all look stupid. We made up the name William Ashbless for this, and then ever since, whenever Blaylock or I have wanted to include a crazy bearded poet in our books, we've used him. For The Anubis Gates I cooked up a thorough biography of Ashbless, in hopes that people would be fooled into thinking he was real."

• Forsake The Sky:
  • p. 135 (Frank Rovzar "picked up a book of Ashbless's poems.")
  • p. 145 (a conversation about poetry includes "...very much influenced by Ashbless, of course...")
• An Epitaph In Rust:
  • p. 44 (mentioned as being read by aspiring poet Thomas)
  • p. 74 (a sketch of Ashbless by Havreville is mentioned hanging in The Blind Moon bar)
• The Drawing of the Dark: pre-prologue page (quoted)
• The Way Down The Hill: p. 6 (Ashbless' Odes is mentioned as a work in Sam Hain's library)
• The Anubis Gates:
  • throughout (character)
  • page prior to "Book One" (quoted)
  • p. 349 (quoted)
• On Stranger Tides: pre-prologue page (quoted)
• The Stress of Her Regard: p. 363 (mentioned as the translator of a work by Francois Villon)
• Last Call: p. 33 (quoted)
• Expiration Date: p. 93 (Guillermo Ceniza-Bendiga - William Ashbless in Spanish, is mentioned as the author of Conjuro del Tobaco)
• Earthquake Weather: p. 20 (Guillermo Ceniza-Bendiga's Conjuro del Tobaco is mentioned again)
• Declare: throughout (Elena Ceniza-Bendiga [relation to William unknown] is the female lead character.
• The Elfin Ship (Blaylock): p. 185 (Jonathan Bing quotes an Ashbless line)
• Homunculus (Blaylock):
  • pre-prologue page (a quote supposedly from Robert Louis Stevenson mentions "a volume of Ashbless' poems").
  • p. 45 (Bill Kraken has a copy of Ashbless' Account of London Philosophers...)
  • p. 55 (...and it saves his life!).
  • etc. (he proceeds to refer to it throughout the story).
• The Digging Leviathan (Blaylock): (character)
• Lord Kelvin's Machine (Blaylock): p. 67 (mentioned as the author of Account of London Scientists)
• The Last Coin (Blaylock): p. 249 (quoted)
• Freedom & Necessity (Steven Brust & Emma Bull): p. 100 (mentioned as a volume in Richard Cobham's library.
• Offering the Bicentennial Edition of the Complete Twelve Hours of the Night: 1785-1985 (hoax prospectus of a William Ashbless collection, with a sample poem, signed by Powers & Blaylock; one signing "William" and the other signing "Ashbless")

For further reading on this character: John Berlyne's Ashbless page has been expanded and contains some great stuff, notably a sketch of Ashbless by Powers and an actual Ashbless poem written exactly as Powers describes it - one line by Powers, the next by Blaylock, and so on. Don't miss it! Also take a look at International William Ashbless Society page. This is a unique tribute to the Ashbless phenomenon by a Powers / Ashbless fan.