From the publisher's website:

Despite running the gamut from A to Z, GURPS Alphabet Arcane couldn't confine all its wonders to one supplement. Fortunately, there's GURPS Alphabet Arcane: Lost Serifs to provide some additional illumination from Stefan Jones (author of GURPS Uplift and MacGuffin Alphabet).

Secrets revealed herein include:

• Expanded information on Condor Boy, a young hero who plays an important part for many of the artifacts from GURPS Alphabet Arcane.
  
• A description of the fallen paradise that serves as the otherworldly realm where some ancient entities reside between visits to our world.
  
• An alternate version of the Fabulous Pavilion that recasts that dreamland party as an actual pocket universe!
  
• Additional material that helps to introduce several items; use as scripts or sources of inspiration for incorporating those items.

If you've already visited the world of GURPS Alphabet Arcane, these pages can help add depth and variations to its items; if you aren't yet familiar with that supplement's charms, GURPS Alphabet Arcane: Lost Serifs is an excellent (and affordable!) introduction. Regardless of how well you know the alphabet, you're bound to learn something new!