I like the physical design of the Classic Traveller books. The 8½×5½ stapled booklet is just the right size, physically, to read easily and carry around, and it opens and lays flat, for easy reference. As far as the information content, the physical constraints of the format provides enough physical space to present a comprehensible amount of information, without enough the temptation to pad the content with irrelevancies.
I also like visual appearance of these books: the black covers of the original books — with white text for the book title and subtitle, and red text and a thick red line for the game title and publisher — were stark, attractive, and stand out even today.
I gather, from comments from Steve Jackson Games, on the comic-book sized booklets they printed for the GURPS Traveller line in the early 2000s, that books of these form factors aren't cost effective for traditional RPG publishers any more. That's a pity.