That's why I've dedicated this series to all the great sketchbooks of fantastic artists. I've saved a lot of sketchbook bookmarks that have inspired me tremendously, that's why I'm sharing my favorites now.
Krato over at ca.org (link)
10 pages, there are some nice studies in there, definitely worth a look!
Sidharth Chaturvedi (link)
Some very nice oil studies in there, as well as graphite and digitals.
If you ever need some motivation to get started with anatomy again, or you just enjoy anatomical studies, then this sketchbook is a great place to start! 2 pages and counting (I think).
Another inspiring thread full of studies.
More will follow!