There is no specific book or brand related to NATA coaching as architecture is rarely sought after. You can go with any of the books you get your hands on. Just check that it has good reasoning and mental ability type questions. If by preparing online, you mean just reading material and watching video lectures, then that would not be enough because you will need practice on paper. Solve as many reasoning and aptitude questions as you can. Buy any related book which is not expensive and has good number of practice questions. You will be good to go.