That are most useful in understanding proportion etc?

If you are starting to get serious about anatomy I would recommend "Atlas of Human Anatomy for the Artist" By Stephen Rogers Peck as a great starting point.

The book is extremely well priced at around $12. It has both drawings, pencil and digital, as well as photos.

It starts out with bones and shows drawings of how this affects movement. Muscles is next with the same type of information. If it helps you there is also names for all the parts of the bone/muscles and text references for where the points meet. There are a few full body examples of where the muscles would be paired with an actual photo of a person in a pose.

Details of the body (Veins, Facial features, navel, ect) are also added. High contrast photos are included then a small section on proportion, on body types, age/sex/race, and the book finishes up with expressions.

It is a very good anatomy book because it touches on all the basics wile providing a great structure that will lead you into more advanced anatomy books. (Ones with more drawing references which you have to know the body pretty well before hand to avoid making mistakes that the artist may have.)

Hope this helps.