I totally understand the desire to read lots of books, but I personally don't understand the urge to own lots of books. Even when you pay for them rather than wait for them to show up at a library, why not sell or trade them when you're done? It feels like a socially acceptable version of consumerism to me with aspects of hoarding, and I suspect that if everyone here posted what they've spent on books it may be received a bit differently.
It depends a lot on whether the books are tools of trade, or forms of entertainment (and, if forms of entertainment, whether they'll be used again and again, or their presence creates a sense of home, etc.).
I own no "entertainment" books (except a couple given as gifts, which are still hanging around because I haven't yet read them, and feel guilty disposing of them without being able to discuss their contents with the givers...). I own several hundred books - close to a thousand- as 'tools of trade', because they aren't easily available in other ways, or I need to refer to them often enough that it would seriously impact my productivity to need to reference them at an institutional library. If I just need to find a half-remembered quote somewhere, Google Books is pretty good. Once I've found that half-remembered quote, and need to review the context to make sure I'm not misrepresenting it, I need the whole text, and it can take months to obtain things stored at an overseas institution - sometimes with a heftier price tag than just owning the thing (my library told me recently that, because a work I need isn't at an institution with which they have a reciprocal lending arrangement, it would be 3000 euro to get a copy!).
I've just designed a sweeping postgraduate course that covers an unusual amount of ground: almost every book came off the shelf for that, either to help me choose readings for my students, or as background research for lectures. I'll be writing a book next term, and I expect to ransack the shelves again while doing that.
That said, some of this depends on the kind of institutional access you have. I'm in a minority any my current university, in terms of how my research fits with the research of others. It just doesn't make financial sense for this university to stock, or maintain subscriptions, to a lot if what I need. They also cap the number of works (electronic or print, local or inter library loan) anyone can have out, at 50 works, and require everything to be returned every few months. They also occasionally tell me I've exceeded their inter library loan cap, so that I can't make further requests until the following year...
By contrast, at a previous university, I could check out infinite works, and didn't need to return them until someone else recalled them - at which point I had to have them back in 48 hours or face punishing fines. I kept around 500 books out at any given time while I was researching. And I was just an undergraduate student!
That's not possible where I am now, so I'll buy anything that I need for long-term course design or research purposes.