To me, every time that I have read the TVD novels and they describe Bonnie's physical appearance, this is how I have always imagined what she looks like. Bonnie, to me, has a very "avant garde"
beauty. When Bonnie's appearance is compared to that of her best friend, Elena, then her beauty is viewed as off-beat
. Bonnie's beauty in my opinion has always been "classic". Elena's beauty has always been superficial, IMO. Elena is the "drop-dead gorgeous" girl with all the right proportions, with golden long hair and eyes that are an interesting blue, etc. Believe it or not, there's an "off-beatness" and yes, even awkwardness
about Bonnie's beauty. She is basically small-chested, very petite in height and she doesn't have an overly developed body the way Elena does. Damon has even described her body and physique as diminutive and underdeveloped, basically referring to her as a "woman-child"
(which in my opinion is the cutest description by Damon, ever ♥). Bonnie, I don't think has ever really felt that she was the beautiful girl but just the "cute" girl who's pretty and dainty. In my opinion, if I had to choose which kind of beauty is most relatable to me between Elena and Bonnie, I would most definitely say Bonnie. Bonnie's physical appearance is not made out to be "perfect" the way Elena's appearance is made out to be. Also, this leads me to strongly believe that Damon is only in love with her (Elena) because of outside characteristics; for shallow purposes such as her appearance (and also because of her strong, compelling aura as a supernatural human and also her supernatural special blood). Nonetheless, regardless of how beautiful Elena is (superficially, I shall add), Damon is still strongly and deeply attracted to Bonnie. Yes, Damon is also sexually attracted to Bonnie. The fact that Bonnie is so underdeveloped in physical appearance only makes Damon more attracted to Bonnie I think. Like I said, Damon is deeply fascinated by and strongly and sexually attracted to Bonnie's innocent and maidenly persona. She has this innocence but to Damon, her innocence is her sexuality; Bonnie's innocence and maidenly-ness is sexy to Damon. Bonnie is not the "drop-dead" gorgeous girl but according to the descriptions in the novels, she is strikingly pretty in the classic unconventional way. Bonnie's beauty is basically the opposite of Elena's beauty (as well as her inner personality and her inner beauty as well but that's a diff. story). Elena has perfect long, straight, golden blonde hair like that of an angel; Bonnie has curly, strawberry red hair which is rather frizzy and frazzled at times. Elena's eye colour is an "intriguing" blue colour (described to be like Lapis Lazuli); Bonnie's eyes are a simple and subtle brown with a lighter hazel rim around the outer edge of her inner eye colour. Elena is of the ideal height for a teenage girl; Bonnie is petite and short in stature and in height. Both Elena and Bonnie have very fair, translucent skin but Bonnie's blue veins and fjords are significantly more visible then Elena's (Damon has taken significant notice of this). What I am basically saying is that, Bonnie is the complete opposite of Elena in terms of outer beauty. Whereas, I have always felt that Elena's beauty is "superficial", I have always felt that Bonnie's beauty is natural, organic and classic. I think that Damon is highly attracted to this, because believe it or not, Damon is actually possesses the same kind of beauty that Elena has; that drop-dead gorgeous, stunning guy that all the girls go bezerko over, while Damon's brother, Stefan (like Bonnie), is more of the "classically handsome" looker (with jet black wavy hair and piercing green eyes).
Also, I would like to add that on the TVD show, I think that Ian is attracted to Katerina (and it shows on-screen, hence why he has so much chemistry with her) because I believe that Ian has a strong attraction and a thing for exotic girls. I remember when Ian was on Smallville, he couldn't stop mentioning how drop-dead gorgeous and "intimidatingly" beautiful and exotic Kristen Kreuk was. (Kristen Kreuk and Katerina Graham are both mixed with Kristen being half Chinese/half Dutch and Katerina being half Polish-Russian/half Liberian). He also said that Kristen (like Katerina) is not the typical-looking girl you find in Hollywood, L.A. (meaning she is exotic and not the stereotypical looking plastic chick with big boobs, and bleached blonde hair). This only makes me think that Ian strongly attracted to exotic girls. He talks very highly of them.
Here is what I have always pictured Book Bonnie to look like: