The spoiler policy on books and stories is very simple here: The American Girl Wiki is a spoiler heavy zone.
Editors are not obligated to mark any pages with "spoiler space" or any notes that the pages have spoilers. Spoilers will occur on any page freely.
To be perfectly honest, there's no reason to mark spoilers with any books.
With the exception of recently released books or series and Girls of the Year, books are WAY past the Internet Statute of Spoilertations of about six months. Movies are always based on the books--even if in the loosest of senses--and are released generally six months or more after books are.
If you don't know by now that Kirsten and the OMFG BEAR ends with a snazzy hat, Addy doesn't get captured by the Confederates and sold down south to Georgia when running away, and Felicity gets Penny back, you are a couple years behind the rest of the people who have read the series. Even the mystery novels, recent releases, and any new items will remain unspoiled.
Long story short: Don't click on book stuff if you haven't read the books.
For spoilers and data on items, please see Rumors, Leaks, and Hearsay.