The value of the books can vary by campaign, material you have and knowledge of GURPS (Btw: Buy the Character generator and get the Lite Version of the rules, suddenly the Crunch is gone)
FirstInis IMHO the least useful of the books. System generation is optimised for GT and uses GT slang (that sadly differs from most other Traveller when it comes to Port classes and other minors) with little additional background on Scouts that is new. The ScoutCruiser and the Courier are nice but GT stats only.
Starports and FarTrader are somewhat useful. Again they use GT profiles and a rather detailed (and complex) trading system. But they give you material on trades, traders and starports that other versions are mostly lacking or that is DGP material and therefor OOP. Some points should be taken with salt (I.e I dropped the "unbribabel officials" faster than a cheap hooker drops her panties)
Alien books are so-so. The one or two major races covered by each book are good and highly detailed even compare to the old GDW material. Some of the Aliens really FEEL Alien with that material. The minor races that are given short intros are so-so ranging from "Oldies but Challenge Goldies" (Hhkar) to "Not designed for Traveller" and when it comes to Darrians YMMV on the tech thing. Stats are for GT but as said translateabel
Rim of Fire, Behind the Claw No comment since both my SM/Norris and SolRim/Solomanie work different from Canon. Even more so post 1116 since I don't use the GT timeline
Starships, Modular Cutter, 600dton So-So useabel. Starships can give some nice ideas for Additions to MT or TNE starship construction but will produce GT ships only. Cutter is IIRC a GT only supplement but easily back-ported or used as an inspiration. Worth for the 90dton and 100dton cutter versions and the various modules alone. 600dton is my favorit ship, the Typ-M Stellar/Bastion in various variants and stated for multiple systems
Nobles The ultimate book on organised crime in the 3. Imperium. From Knight to Emperor they are all well defined and described with examples and lots of fluff on them and their (non-noble)toadies. Some toys (Yachts, MegaCorp Courier ship) with GT-stats and some sample characters/templates for GT. Both call "loot me, adapt me" Again written through the "perfect 3I" lenses instead of the "rotten 3I" lens of MT/TNE. Nobles is one of the reasons to use GURPS for Traveller
Humanity So-So book about various Human sub-species. Some are interesting, some are classics (Answerin, Genoee) some are NPC material. All in GT stats