I have bought tons of 11+ books and non-11 plus,piles,second-hand and many new...but I have just dug out Collins Revision,Key Stage 3 Maths,level 5-8,large red book with about 230 pages. It is packed with maths tutorial (brief),then examples and then questions from a large number of maths topics,most of which appear in 11+ even though it is not an 11+ book.It has equations, geometry, data,radio,proportion.... tons. It also has answers at the back. I wish we had started this book earlier as it is great compared to the tiny,overpriced official 11+ books that give no explanation and for some you need to pay another fortune for the separate answer booklet.
Also,I have found so useful the small old 11+question book (blue and purple,the same thing,different colour) "Genuine Exam Questions from Yesteryear" I bought on Ebay for a couple of pounds. It is full of 11+ questions (and answers) from 30-40 years ago,but most are still in 11+ tests,like speed,proportion ,etc...Very useful and with a huge number of questions,even general knowledge,just the stuff you find in 11+ books today.Tons of small tests with answers in it.
Also,the free Maths Centre website has explanations,questions,answers,just great,all free.
I find many 11+ books a rip off,some have no answers, or you need to pay extra, most have no explanation,many are quite small yet not cheap.
Any other,not so obvious,recommendations for DIY 11+?