The Osprey book correctly describes an ehuatl, which is a tunic or vest, whereas the tlahuiztli are full body suits. In the Florentine Codex, there's this depiction of both an Eagle and Jaguar warrior dressed in tlahuiztli, which you can see covers them head to ankle. Other depictions, like this one from the Codex Mendoza, do not show an included headpiece and are of much simpler design. Given that the Codex Mendoza is a tribute list, and that tlahuiztli are listed in it as tribute items, we can make some inference (though nothing certain) that the pieces shown in it were "off the rack," rather than custom made for a particular individual. There was undoubtedly a high amount of variation in the suits though.

There's a clue in the Osprey plate about the distinction between tlahuiztli and ehuatl. Note the two men depicted wearing ehuatl are literally the two most elite persons among the Aztecs, a tlatoani and cihuacoatl (Motecuhzoma Ilhuicamina and his brother Tlacaelel, in this case). Hassig, in Aztec Warfare, notes ehuatl are associated with elite figures and could almost be seen as more ceremonial and symbolic than functional (having been eclipsed in utility by tlahuiztli). He notes they are often found depicted at Teotihuacan and Chichen Itza, but in Aztec times they are basically only shown to be worn by elites (and most often elites from the Acolhua-Tlaxcala corridor), who theoretically would no longer be charging forward on the front lines. In fact, the most famous depiction of ehuatl is on Nezahualcoyotl, who is shown not only dressed in ehuatl, but is also carrying a drum on his back used to signal troop movements during battle. The portrayal of elites in ehuatl could be an artistic trope, and they may in fact have used tlahuiztli in actual combat, but the former piece of armor is certainly something that has an association with high rank.