应该是：He seemed to be reading a novel when I came in.当我进来时,他似乎正在读一本小说.
seemed to do sth:“似乎做...”,不能表达“正在进行..."、"正在做...”之意,“似乎在读”有“似乎正在做...”的意思,要用不定式的进行时态,所以应该填："seemed to be doing sth",具体到这句话中,read就要加 ing,所以应该是：“seemed to be reading ” .完整句子见上面我的回答.