Snacking Cakes (By Yossy Arefi) is full of recipes for single layer 8" square cakes, many of which include instructions to change it up to 9" round, loaf, bundt or cupcakes. I have also had good luck scaling her cakes a 6" round. I have read alot about this online but wish I could find a hard copy book dedicated to 6" cakes. Her newer title (Snacking Bakes) also includes a similar style cake chapter (single layer 8" x 8" cakes with various other pan options). This book also has chapters on cookies and bars/brownines Her recipes are mostly one bowl and really do come together quicky.
Snackable Bakes (by Jessie Sheehan) also has some smaller sized desserts but is certainly not as dedicated to the idea of smaller yields as the above titles. This one is organized by ingredient/flavour profile...chapters include nuts, fruit, chocolate, vanilla, sweet & salty etc.
Of course there are lots of EYB member notes available for a ll 3 titles to give you an idea :)
Question - are you happy with Kimber's Small Batch Bakes? I have it on my wishlist.