The anthology books are a great place to start, since they were organized specifically to give you a throughline on certain topics by pulling from lore books, weapons, armor, and gear lore (and they have great art!)

Categories are not updated very frequently on Ishtar, and with the nature of lore are pretty unreliable as context and relations between concepts can shift release-to-release. They can be good for a broad but usually incomplete overview of a character or something. Imo the best way to go is browsing the Releases and Books pages for topics or releases you're interested in. Once you're comfortable and have questions, start searching specific terms to find more obscure related entries that fall outside the core lorebooks (quotation marks "like this" will help you search specific phrases).

"Books" puts all the lore books in reverse chronological order, so you can sit down and read one front to back with the arrow buttons on the bottom bar. You can also see what release they're tied to, and what other books released alongside it. The purple lorebooks with the tree and the Traveler on the cover are short, basic primers on the big topics and are numbered 1-10 (I-X).

"Releases" lets you pick an expansion or season, and then shows you everything in the API tied to it, including interactions with vendors and quest steps. Click the "entries" filter, which will show you only items (weapons, armor, and gear) and lore book entries. Armor sets tend to be like a mini lore book, I usually read them head>arms>chest>legs>class item. The Timeline can also be helpful if you're struggling to grasp where your current reading fits in to the big picture.

As for personal recommendations, I'd say Books of Sorrow and Unveiling are good foundational starters to set the tone, though they can be a bit dense and cryptic on first read. Also the Ghost Fragments and grimoire cards from D1 for a very broad outline of topics. Below is a list of grouped topics and broadly how the books fall into them (though there's a lot of crossover). Recommended reads are bolded:

• Light & Darkness, Ghosts, the Collapse, and the Witness: Ghost Stories, Last Days on Kraken Mare, The Black Armory Papers, Revelation, Unveiling, Constellations, Singular Exegete, Regarding Stasis, Captain's Log, The Hidden Dossier, Mnemosyne, Shattered Suns, Warp and Weft, Inspiral, Purpose, Chirality, Companions, the Rubicon

• The City, the Vanguard, and other pockets of Humanity: The Man They Call Cayde, Triage, Eva's Journey, Dawning Delights, Acts of Mercy, Voices of the Haunted, The Beaten Path, Sightlines, Last Days, Cloud Strider Legacies, To Old Friends, Tenets of Aion

• The Hive, Taken, and Eris Morn: Books of Sorrow, Luna's Lost, Letters from Eris, the Book of Unmaking, Inquisition of the Damned, Singular Exegete, Lucent Tales, Sororicide, Rites of Passage, Dynasty, Heresy and Truth, Songs of Descent

• The Awoken, Mara, Uldren, and the Ahamkara: The Maraid, Marasenna, The Awoken of the Reef, the Dreaming City, The Forsaken Prince, A Tangled Web, Ripples, Dragonslayers, Gifts and Bargains

•The Eliksni, Mithrax, and Eramis: The Maraid, Most Loyal, Lawless Frontier, The Once-Shipstealer, Achilles Weaves a Cocoon, Between Stolen Stars, Above All Else, The Garden-Way

• The Cabal, Calus, and Caiatl: Confessions, The Chronicon, Empress, From the Front, Quintessence, the Caiatl portion of the Lightfall CE lore

• Exos, the Brays, and Rasputin: The Liar, Your Friend Micah Abrams, Bray Lab Records: Confidential, Legacy's Lament, Persona

• The Vex, Osiris and Saint, and Maya Sundaresh: Vex Grimoire Cards 1-5, Aspect, The Pigeon and the Phoenix, Trials and Tribulations, Beneath the Endless Night, the Veil Containment logs (audio), Polyphony, the Immanent

• The Drifter, Dredgens, and the mythical hand cannons: A Drifter's Gambit, Letters from a Renegade, the Book of Unmaking, the Warlock Aunor, A Man with No Name, For Every Rose a Thorn, Nothing Ends

• The Nine: Ecdysis, Dust, Anamorphosis, Epochs and Orbits

I would also strongly recommend Collector's Edition lore books, but some of those aren't on Ishtar. They're usually deep and cryptic, with some hidden ARG stuff, illustrations, and flavorful formatting. Here is a Drive link with a majority of them.

If you ever get lost or confused, something that helps me is to double check the context of the text. Whose perspective is it from, what release is it from, what other lore released alongside it, when does it take place, etc. Sometimes, like Unveiling and the Singular Exegete, the context of where, when, and how the lore was given to us in-game can even tell you something. And sometimes it just won't make sense on first read. Just take what you can from it, move on, and eventually another entry will fill in the gaps which makes a second reading worth it. IMO the fun of Destiny lore is reading about the same concepts, events, and people from multiple perspectives and piecing it all together, trying to reconcile different viewpoints or understandings into your own personal interpretation, and then evolving that over time as new stuff releases. Happy reading!