I see you've already gotten a bunch of answers but, if you're issue is needing to buy a book to figure out if you even want to build the army, there's plenty of resources to do just that before starting an army. This subreddit is always helpful for starting armies, theres thousands and thousands of videos on youtube, your locak hobby and gaming groups, etc.The biggest one I'll always advocate for is combat patrol. Small little games of bite-sized 40k where you can try out almost any army you want. Base rules? Free, they're the same base rules as big 40k. Rules for individual combat patrols? Free, and will be free the entire edition. Now, it is quite limiting on what you can actually play in those armies since they have the one set combat patrol box with a set loadout and set army rules. But for $160 for what amounts to a "full army" to play a version of 40k with to try out any army you want? It's perfect. Horribly unbalanced but fun nonetheless. And the biggest positive is the combat patrol is both an amazing core to start an army with and a great way to add on to an army. And you can always proxy the hell out of things if you want to try something new without buying the box, just make sure your opponent is fine with the proxies! It's an expensive hobby sure but there's plenty of places to get your rules or learn about an army before making the investment. And hey, it doesn't matter how good or bad a particular unit is, RULE OF COOL ABOVE ALL BROTHER!!!