This 1980 study of Robert A. Heinlein and his work is still valuable, though Heinlein went on to publish another four novels, including a very solid SF adventure novel, Friday, and an amusing, Hugo-nominated fantasy, Job. Nothing about that late body of work, however, invalidates Franklin's approach.
Franklin is especially good at explicating Heinlein's fiction against the background of socio-political currents in the US during the period from the 1930s to the 1970s. Sometimes his readings seem politicized to the extent of overreaching and distorting, but Franklin usually wears his Marxist political ideology lightly (and where it does show through, it leads him to opinions, such as those relating to the ills of post-war US foreign policy, that actually strike me as persuasive).
Some of the gems include a careful account of why I Will Fear No Evil (perhaps my least favourite Heinlein book, and Franklin seems to dislike it even more) found and entertained a large audience, and a detailed, relatively sympathetic, thematic analysis of The Number of the Beast. The latter is a book that I should dislike on principle, with its endless self-reference and apparent self-indulgence... and yet, the characters are fun and likeable, and the story can be enjoyed on its own terms if you're sufficiently a fan of Heinlein's earlier fiction and the other SF and fantasy worlds that are endlessly referenced. Someone approaching it without that background would, I imagine, be hopelessly lost and quickly out of patience with it.
I wasn't sure how useful a 35-y.o. critical study of Heinlein would be. I'd dipped into Franklin's book many years ago (I think... I read a lot of SF criticism in some earlier phases of my life), but it made no lasting impression. However, it does offer thoughtful, plausible perspectives on Heinlein's fiction, identifying elements that might be elided in less historically and politically savvy readings. It is written straightforwardly - to provide information and express ideas, rather than impress with its author's cleverness and erudition - and it will have insights for almost any student or fan of Heinlein's work. It's fitting that this is still considered an important book its field.