I picked up Between Planets because it happened to be on the shelf of my university library, not because I have been reading/re-reading Heinlein's books in any systematic order (beyond concentrating, for the moment, on his work prior to the 1960s).
This is, however, a fine example of Heinlein's juveniles (today we'd use the expression "Young Adult novels") published by Scribner's in the 1940s and 1950s, and aimed largely at an audience of teenage boys. Between Planets first appeared in 1951 (initially in serial form and then in book form from Scribner's). 1951 was relatively early in the author's career when viewed from today's perspective, but it was already a few years after the end of the John W. Campbell "Golden Age" of SF. Between Planets was the fifth of the Scribner juveniles, and by 1951 Heinlein's career was well established, even if his most important books (such as Stranger in a Strange Land and The Moon is a Harsh Mistress) were yet to come.
The story is focused on the efforts of a teenager, Don Harvey, to get home to his parents on Mars. Don is attending high school on Earth when war breaks out - as settlers on Venus rebel against Earth's oppressive empire. This rebellion is compared throughout the narrative to the American War of Independence, and we are invited to take the rebels, pretty much uncritically, as the good guys. Some individuals on the rebel side do show flaws, but the rebel forces display decency and restraint throughout the conflict, while their Federation enemies are unscrupulous and even cruel (albeit sometimes cunning and charming).
Don gets caught up in complicated events that involve a long detour to Venus, and extensive efforts to make some sort of life there, while still working out how to get in contact with (and eventually return to) Mars. Not surprisingly, for this kind of story, he ends up playing a vital role in the revolution.
There's little not to love about this well-told tale, which features physically impressive (but always reasonable and kindly) "dragons" from Venus, a lost super-civilization to investigate on Mars, much discussion of revolutionary strategy, and plausible descriptions of interplanetary routes and the required futuristic infrastructure. Thematically, the novels displays Heinlein's concerns with freedom, choice, and the value of all intelligence - human or otherwise.
Between Planets has, of course, dated in many ways. Most obviously, it shows Venus as an inhabitable world, while a more modern version of the story would doubtless offer more to female readers and pay far more attention to the female characters (that said, Don's love interest, Isobel, does get some good showings during various incidents on Venus). If you cannot imaginatively place yourself in the ethos of 1950s SF, the story will not entertain and move you.
Otherwise, it's another good example of why the Heinlein juveniles are loved and revered by many older readers... and can still attract an audience. They are well-made, cleverly worked out, optimistic, and (perhaps above all) respectful to young readers. These novels were already long in print even 40-something years ago, when I was first introduced to them, but if you can get into the right frame of mind they continue to offer much enjoyment.