The Fire and The Disappeared are books that could be read as stand-alones, but their characters overlap in The Disappeared as one of The Fire's MCs brings together the MCs from The Disappeared, so it's better to read The Fire first to follow the context in chronological order.
The Moments, Reality Check, Love Forged, and The Show Must Go On are not connected to any other book, so they can be read in any order you choose.
The Moments also includes two paths of events, an alternative to how the lives of the characters play out.
The Meet Cute Café features 8 love stories that are interconnected in one book.
At the center of the book is the café itself that unites all the couples' stories together.
Pride Festival is a YA novella, part of I Heart SapphFic Pride Collection as a themed book about Pride and is not connected by characters and/or universe to the books in the collection.
Fresh Start is an introduction to the series. Can be read as a stand-alone as the none of the MC's in the following books are featured in it. The MCs don't start in Chicago, but they later move and are heavily featured in the series.
The Best Lines introduces most of the MCs in the series, two of which get their own book, Just Tell Her.
Just Tell Her is based on 2 BFFs introduced in The Best Lines, and the characters are heavily featured in both books.
Love Walked into The Lantern is based on MCs featured in Just Tell her, and it tells their story.
One of the featured characters in the Chicago Series moves to San Francisco in the last book, which gives way to a new branch, The San Francisco Series.
SAN FRANCISCO / TAHOE SERIES:
Checking the Right Box (SF Series #1)
Keep Tahoe Blue (Tahoe Series #1)
Keep Tahoe Blue is introduced mid-series for San Francisco.
The events in the book take place before book #2 in The SF Series, and therefore, chronologically, is recommended for reading after Checking the Right Box and before Macon's Heart as one of the four friends from Checking the Right Box moves to Lake Tahoe, starting The Tahoe Series.
Macon's Heart (SF Series #2)
This Above All (SF Series #3)
The San Francisco Series ends with This Above All, and we are back to Tahoe:
Time of Day (Tahoe Series #2)
The Perfect View (Tahoe Series #3)
Begin Again (Tahoe Series #4)
Let Go slightly overlaps with Macon's Heart through Macon Greene from The San Francisco Series, and for Macon's back story it's better to read Macon's Heart first.
This series doesn't overlap with other series/books.
Despite the publication date of All the Love Songs being first, for chronological order, it's better to read No After You first as it's a prequel about two heavily featured characters from book 2.
All books are very interconnected, and it's not recommended to read them out of order.
What Happened After for the series should only be read after completing all 6 books.