Okay, this is one of my favorite rabbit holes to dive into — shapeshifter romances are such a comfort food shelf for me. If you want a strong, long-running series with politics, pack dynamics, and emotional stakes, start with Nalini Singh's 'Psy-Changeling' universe (beginning with 'Slave to Sensation'). Her world-building around packs and psychic societies still hooks me every reread.
If you like a grittier urban-edge with a focus on a single heroine who navigates packs and secret politics, Patricia Briggs' 'Mercy Thompson' books are perfect; start with 'Moon Called'. Kelley Armstrong's 'Women of the Otherworld' (the Elena Michaels 'Bitten' storyline) gives a darker, investigative vibe and great werewolf lore. For lighter, goofy fun, Shelly Laurenston's 'Pride' books are hilarious and full of lovable bear- and cat-shifters.
Beyond those big names, check out Eileen Wilks' 'World of the Lupi' (romantic paranormal mystery), Rachel Vincent's 'Shifters' series (young-adult leaning but satisfying), Lora Leigh's 'Breeds' (genetic shifters and heat-heavy reads), and G.A. Aiken's dragon-shifter romps (start with 'Once a Dragon' if you want laugh-out-loud fantasy). Kresley Cole's 'Immortals After Dark' mixes lycans and other mythic shifters with spicy romance. Each author brings a different tone — angsty, cozy, comedic, or steamy — so pick the mood you want and dive in. Happy prowling!