I'd like to preface my response by saying that I have an English degree and am an avid reader. I love books, but I'm not a huge fan of the book in lieu of cards suggestions. I don't buy cards as I see them as a complete waste of money, so this would be an additional expense for me. I prefer to give shower gifts that are more needed like diapers, a Bumbo, teething meds/diaper rash cream if it's on the registry, since it seems like most people tend to go for the cutesy items like clothes, which are nice, but you often end up with so many and not much of the equipment and other necessities. As a new mom, I much preferred picking out my own baby clothes and books as what I liked often differed from others' tastes (and around here at least, nobody buys the clothes or books you put on the registry; they gift what they think is cute).
I've also attended a couple of showers that have had this book request and seen the little variance in books given. We didn't even do the book request at my showers and I still ended up with (between both showers) 7 copies of Goodnight Moon, 4 of I Love You Forever, and triplicates of Green Eggs and Ham and The Hungry, Hungry Caterpillar. So many people seem to give these books, shower book request or not.
For someone that has moved several times (including multiple out of state moves) since having kids, I flat out don't have room for duplicate copies of books, nor do I really keep most books once the kids have outgrown them (again, no room). It definitely makes it difficult when people inscribe things in the covers of books as they can't really be exchanged or given away if duplicates or outgrown. I just threw away about a dozen books that my mom had given to the kids or I that she wrote a message in or even glued pics of us to the inside cover of. I seriously felt sick throwing away books, but it was stuff we have never and would never read and I couldn't justify keeping them and couldn't get the words/pics off without tearing the cover/first page out.
For both of my showers, the host did have a cute photo mat for guests to write messages to the baby. We still have both of those and I really like them because all the messages are together, it's small enough to be easily kept through the years, and serves a double purpose as holding a picture.
I had a baby shower for each of my kids. They were five years apart and girl, then boy. The shower for my first was thrown by co-workers though since I lived on the other side of the country from family. A few family members sent gift cards, but none went to the actual shower. By the time we had our second, we had lived in 3 different states and didn't really have any baby stuff anymore (most of what we had with our first was used when we got it and falling apart by the time she was done with it so it wouldn't have survived the moves had we tried to bring it all with us). The shower for my second was all family and really helped us get some of the necessities we needed.
I will add though that I also never had a graduation party or any of the marriage related parties/gifts (married cross-country in the court house), so according to DIS party standards, my extended family got off pretty easy on gift-giving occasions with me.
My husband did an alternate form of this when I was pregnant with each of our kids. He held a diaper shower and invited his guy friends (and my brothers when they lived nearby for our second). Basically, he told the guys to each bring diapers (any size, any brand, any amount) and come hang out at the house. He grilled food for everyone and provided alcohol and everyone hung out, played poker, and had a fun night while I stayed the night with friends (first shower) and my mom (second shower).
It was nice for him to have a night with the guys and it helped to build a stash of diapers. Any store will exchange unopened packs of diapers so if there was a brand/size I couldn't use, a quick trip to
Walmart allowed me to swap them out.
We did that at my second shower, but also used the envelopes to raffle off prizes. Every guest received a B&BW soap as a party favor, but we raffled off a couple other gifts since I really didn't want to do traditional shower games. I know some people find it tacky to address their own envelope, but it's super helpful for the mom-to-be when writing thank you notes to not have to track down addresses and have their possibly swollen hands cramp up even more with addressing dozens of envelopes.