How did Helen need the books so badly?

Interesting questions. Helen did not choose this time to let Meg know about her pregnancy - as you point out, she was tricked. She was not expecting to have to see anyone.

I think the most interesting question is why the books are so important to her. Themes that run throughout both the book and the film (which stays very true to the book) are home and belong (identity) and property. The story is about England and who symbolically "owns" it - who exemplifies the soul of the nation. Although the Schlegels are part German, I expect Forster considered them more English than the Wilcox's (with the exception of the first Mrs. Wilcox, who spiritually is clearly in the Schlegel camp) who are really more international capitalists. The Schlegels are tied to their home, whereas the Wilcox's are not. Note how the Schlegels are so undone by having to leave the home they have been in their whole lives, and are searching for another; it is fitting that, in the end, they got Howard's End, because I think that estate really represents England. The Wilcox's, on the other hand, don't really have a home (again, with the exception of Mrs. Wilcox, who is forever tied to Howard's End); having plenty of money, they move from residence to residence, but never feel any connection to them. That is probably why Mrs. Wilcox wants to leave Howard's End to Margaret Schlegel. The Wilcox's did not really like or want Howard's End; still they thwart Mrs. Wilcox's attempt to leave it to Margaret, because as rapacious capitalists, they are greedy and jealous of everything they have, even when they don't actually want it. This accounts for the absolute refusal Mr. Wilcox makes to Margaret's request to let Helen stay there just ONE night. Back to the books. All the Schlegels are put out by the loss of their home, and their home comprises also their belongings. Margaret mentions many times how she cannot wait to get here things. Unfortunately for Mr. Wilcox's efforts to preventthe Schlegels from getting Howard's End, he made the mistake of putting their belongings there, and it is immediately seen by the Schlegels, and the housekeeper, how remarkably suited they are to the house. So, almost by accident (a pragmatic solution of where to store the Schlegels' things until they are resettled) Mr. Wilcox creates a new home for them. So Helen is missing her things, and while in the emotional state that comes with pregnancy and being all alone, it would stand to reason that Helen really wanted her things, here little piece of home. Once she gets to Howard's End, she feels at home and wants to stay there one night. Mr. Wilcox is prescient (but for the wrong reason - he just suspects everyone wanting to get his property) in saying, if you let them stay one night, they will never leave. And they never do.

That is my take on the books. They have a broader connection to their sense of identity tied to home and the problem throughout the story of the Schlegels' need to find a new one.