Andrew's bath / David McPhail.
Andrew doesn't like baths until one night when he imagines all sorts of animals in the bath with him.
Age 2+

Squeaky clean / Simon Puttock & Mary McQuillan.

Three little pigs who dislike bath time grow to love it after Mama Pig makes it fun.
Age 3+

I don't want to have a bath! / Julie Sykes

Humorous story about a tiger who doesn't want to have a bath, but eventually finds out what fun it can be.
Age 3+

Big Smelly Bear / Britta Teckentrup

Dealing with the sensitive subject of personal hygiene/body odour, this is well handled and has been depersonalized by using bears.  A fun approach but still gets the message across.  
Age 3+

When a book might help

Sharing stories about various situations can put a child’s mind at ease and make transitions and new experiences easier to cope with and more understandable.