In Ptolus, the Delver's Guild is an organisation that supports adventurers exploring the wonders underneath the city. Whatever your party likes to do in the city, they probably will have at least occasional dealings with the Guild. However, during the development of the Ptolus: City by the Spire sourcebook, The Delver's Guild was also a website where author Monte Cook shared a series of articles designed to fit into the city with those who were interested in the project. This work is a collection of those articles.
The Introduction: Welcome, Delvers covers the actual organisation in Ptolus. It describes their offices and the people the party are likely to meet there - there's even a picture of the sign over the door! The Guild offers membership at various levels, maps and other resources and even 'retrieval insurance' - they employ experienced delvers to rescue people who have got into trouble. Delvers seeking work can do a lot worse than sign up for a retrieval team, too. Throughout the book, everything's cross-referenced with the Ptolus: City by the Spire source book to make it even more useful.
Next, Chapter 1: Deities presents additional information about many of the deities originally listed in the main sourcebook and plenty of other ideas about religion that you may choose to incorporate into your game. Of course, if you've dropped Ptolus into your own campaign world, it's perfectly possible to substitute deities of your own for the Ptolus ones.
Moving on, Chapter 2: Locations provides even more descriptions of places to visit to add to the hundreds in the core book. They are all cross-referenced to make it easy to find where they are to be found in the city. Sketches, coats of arms, and all manner of other snippets help to bring them to life. A couple of halfling cabinetmakers with a hidden sideline in guiding people to secret places for a fee. There are generic shop layouts you can use for anything you need to add, a good boarding stables for adventurers from out of town to board their horses whilst they go delving, taverns and more - even a nice-sounding bar and restaurant catering to wizards! Ideas for adventures involving these locales are included.
Next comes Chapter 3: NPCs with a horde of new folk to populate the city and interact with the party (when not busy about their own affairs, of course). Each comes with statblock, background, often a sketch, and other information - perhaps where they live or work (thus adding another new location as well!). Some can be useful or just plain interesting to meet, others you might hope to never encounter.
Finally, Chapter 4: Tips, Tales and Treasures. Here we find new rules materials, new monsters, new spells... new anything that involves game mechanics. The author explains that everything presented here (and in the equivalent section of the Ptolus sourcebook) has been developed organically, as part of the setting. There are also adventure ideas, hints and tips on running games set in Ptolus. Languages, the city layout - it is NOT flat! - and much, much more. Random tables for stuff you find in various places from chests and bags to dungeon rooms. Treasures that are embedded into the city and its denizens, with coherent backstories that make them far more interesting than their monetary value.
If you like Ptolus, this is a treasure-trove to dip into again and again.